netlink: Make nl_sock_transact() always return a reply on success.

netlink: Make nl_sock_transact() always return a reply on success.

Until now, if nl_sock_transact() received a reply that merely acknowledged
success, without providing any other payload, it would return success but
not provide the reply to its caller.  This is inconsistent and could easily
cause a segfault in a caller that expects to see the reply on success, if
kernel behavior changed, for whatever reason, so that a request that
previously returned data now just returns an acknowledgment.  In practice
this kind of change should never happen, but it is still better to handle
it properly.