netdev-linux, netdev-bsd: Make access to AF_INET socket thread-safe.

netdev-linux, netdev-bsd: Make access to AF_INET socket thread-safe.

The only uses of 'af_inet_sock', in both drivers, were ioctls, so it seemed
like a good abstraction to write a function that just does such an ioctl,
and to factor out shared code into socket-util.

Signed-off-by: Ben Pfaff <blp@nicira.com>
CC: Ed Maste <emaste@freebsd.org>