Kernel: Fix TCP sending

- Fix race condition when adding packet to send buffer before other end
  has acknowledged it
- Allow sending multiple packets before receiving ACK for previous ones

kernel/include/kernel/Networking/TCPSocket.h

@@ -113,6 +113,7 @@ namespace Kernel
 			bool						has_ghost_byte { false };
 
 			uint32_t					data_size		{ 0 }; // number of bytes in this buffer
+			uint32_t					sent_size		{ 0 }; // number of bytes in this buffer that have been sent
 			BAN::UniqPtr<VirtualRange>	buffer;
 		};