Kernel: Convert KBuffer::copy() => KBuffer::try_copy()

This was a weird KBuffer API that assumed failure was impossible.
This patch converts it to a modern KResultOr<NonnullOwnPtr<KBuffer>> API
and updates the two clients to the new style.

Kernel/Net/IPv4Socket.h

@@ -110,7 +110,7 @@ private:
         IPv4Address peer_address;
         u16 peer_port;
         Time timestamp;
-        Optional<KBuffer> data;
+        OwnPtr<KBuffer> data;
     };
 
     SinglyLinkedListWithCount<ReceivedPacket> m_receive_queue;