Kernel: Add on_receive callback to NetworkAdapter

Author: https://github.com/deoxxa
Commit: 36d349f7a7
Pull-request: https://github.com/SerenityOS/serenity/pull/499
Reviewed-by: https://github.com/awesomekling

Kernel/Net/NetworkAdapter.cpp

@@ -88,6 +88,8 @@ void NetworkAdapter::did_receive(const u8* data, int length)
     m_packets_in++;
     m_bytes_in += length;
     m_packet_queue.append(KBuffer::copy(data, length));
+    if (m_on_receive)
+        m_on_receive();
 }
 
 Optional<KBuffer> NetworkAdapter::dequeue_packet()