What is claimed is:1. A firewall control device configured to control a plurality of firewall devices provided between a core network and a plurality of sub-networks respectively, the plurality of firewall devices being configured to discard received data based on discarding flow entries defining discarding of data, the firewall control device comprising:a memory; anda processor coupled to the memory and configured to:receive, from the plurality of firewall devices, data amount information indicating an amount of the data discarded in the plurality of firewall devices, based on the discarding flow entries, respectively and node information indicating a transmission source node of the discarded data,identify, based on the data amount information and the node information, data flows including the discarded data which are transmitted from an information processing device indicated by the node information as a single source device of the data flows and of which total sum of the amount of the data discarded at the plurality of firewall devices respectively exceeds a threshold value, andset, in a first firewall device which is included in the plurality of firewall devices and which is provided between the core network and the information processing device, a first discarding flow entry defining discarding of data of the identified data flows.2. The firewall control device according to claim 1, whereinthe memory is configured to store therein the discarding flow entries set in the plurality of firewall devices, andthe processor is further configured to:modify the node information of a second discarding flow entry to the node information of the identified first data flow when the second discarding flow entry specifying a processing method contradictory to a data processing method specified in the first discarding flow entry is stored in the memory, andset the modified second discarding flow entry in the first firewall device.3. The firewall control device according to claim 2, wherein the node information further indicates a transmission destination node of the discarded data.4. The firewall control device according to claim 3, wherein the processor is further configured to:determine a number of the discarding flow entries for the first data flow in which the total amount of the discarded data for each combination of the transmission source node and the transmission destination node exceeds the threshold value,determine a number of the discarding flow entries for a second data flow in which the total amount of the discarded data for each of the transmission source node exceeds the threshold value,select a data flow having a smaller number of the discarding flow entries, from the first data flow and the second data flow,generate a discarding flow entry specifying discarding of data of the selected data flow, andset, as the first discarding flow entry, the generated discarding flow entry in the first firewall device.5. The firewall control device according to claim 1, wherein the processor is further configured to:generate an entry setting request message including information of the first discarding flow entry, andtransmit the entry setting request message to the first firewall device.6. A method using a control device configured to control a plurality of firewall devices provided between a core network and a plurality of sub-networks respectively, the plurality of firewall devices being configured to discard received data based on discarding flow entries defining discarding of data, the method comprising:receiving, by the control device, from the plurality of firewall devices, data amount information indicating an amount of the data discarded in the plurality of firewall devices, based on the discarding flow entries, respectively and node information indicating a transmission source node of the discarded data;identifying, by the control device, based on the data amount information and the node information, data flows including the discarded data which are transmitted from an information processing device indicated by the node information as a single source device of the data flows and of which total sum of the amount of the data discarded at the plurality of firewall devices respectively exceeds a threshold value; andsetting, by the control device, in a first firewall device which is included in the plurality of firewall devices and which is provided between the core network and the information processing device, a first discarding flow entry defining discarding of data of the identified data flows.7. The method according to claim 6 further comprising:storing, by the control device, the discarding flow entries set in the plurality of firewall devices;modifying, by the control device, the node information of a second discarding flow entry to the node information of the identified first data flow when the second discarding flow entry specifying a processing method contradictory to a data processing method specified in the first discarding flow entry is stored in the memory; andsetting, by the control device, the modified second discarding flow entry in the first firewall device.8. The method according to claim 7, wherein the node information further indicates a transmission destination node of the discarded data.9. The method according to claim 8 further comprising:determining, by the control device, a number of the discarding flow entries for the first data flow in which the total amount of the discarded data for each combination of the transmission source node and the transmission destination node exceeds the threshold value;determining, by the control device, a number of the discarding flow entries for a second data flow in which the total amount of the discarded data for each of the transmission source node exceeds the threshold value;selecting, by the control device, a data flow having a smaller number of the discarding flow entries, from the first data flow and the second data flow;generating, by the control device, a discarding flow entry specifying discarding of data of the selected data flow; andsetting, by the control device, as the first discarding flow entry, the generated discarding flow entry in the first firewall device.10. The method according to claim 6 further comprising:generating, by the control device, an entry setting request message including information of the first discarding flow entry; andtransmitting, by the control device, the entry setting request message to the first firewall device.