Kernel: Return the correct result for FIONREAD on datagram sockets

Before this commit, we only checked the receive buffer on the socket,
which is unused on datagram streams. Now we return the actual size of
the datagram without the protocol headers, which required the protocol
to tell us what the size of the payload is.

Kernel/Net/IPv4Socket.h

@@ -84,6 +84,7 @@ protected:
     virtual ErrorOr<size_t> protocol_send(const UserOrKernelBuffer&, size_t) { return ENOTIMPL; }
     virtual ErrorOr<void> protocol_connect(OpenFileDescription&, ShouldBlock) { return {}; }
     virtual ErrorOr<u16> protocol_allocate_local_port() { return ENOPROTOOPT; }
+    virtual ErrorOr<size_t> protocol_size(ReadonlyBytes /* raw_ipv4_packet */) { return ENOTIMPL; }
     virtual bool protocol_is_disconnected() const { return false; }
 
     virtual void shut_down_for_reading() override;