Kernel: Increase IPv4 buffer size to 256kB

This increases the buffer size for connection-oriented sockets
to 256kB. In combination with the other patches in this series
I was able to receive TCP packets at a rate of about 120Mbps.

Kernel/Net/IPv4Socket.h

@@ -110,7 +110,7 @@ private:
 
     SinglyLinkedListWithCount<ReceivedPacket> m_receive_queue;
 
-    DoubleBuffer m_receive_buffer;
+    DoubleBuffer m_receive_buffer { 256 * KiB };
 
     u16 m_local_port { 0 };
     u16 m_peer_port { 0 };