netlink-socket: Reduce nl_sock_recv() from 2 (or more) system calls to 1.

netlink-socket: Reduce nl_sock_recv() from 2 (or more) system calls to 1.

Until now, each attempt to receive a message from a Netlink socket has
taken at least two system calls, one to check the size of the message to
be received and a second one to delete the message from the socket buffer.
This commit switches to a new strategy that requires only one system call
per message received.

In my testing this increases the maximum flow setups per second by a little
over 10%.