netlink-socket: fix typo to get_sock_pid_from_kernel()

A typo crept in while respinning get_sock_pid_from_kernel() in the previous
patch. Fixing it now. Also, get_sock_pid_from_kernel() doesn't need an OUT
argument. Fixing that too.

Signed-off-by: Nithin Raju <nithin@vmware.com>
Signed-off-by: Ben Pfaff <blp@nicira.com>