netlink: avoid putting empty mbufs on the socket queue

netlink: avoid putting empty mbufs on the socket queue

When processing incoming Netlink messages in nl_process_nbuf() kernel
always allocates a writer with a buffer to put generated reply to.
However, certain messages aren't replied.  That makes nlmsg_flush()
to put an empty buffer to the socket.  Avoid doing that because avoiding
is much easier than dealing with empty buffers on the receiver side.