lockd: Specify address family for source address

Make sure an address family is specified for source addresses passed to
nlm_lookup_host().  nlm_lookup_host() will need this when it becomes
capable of dealing with AF_INET6 addresses.

Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: J. Bruce Fields <bfields@citi.umich.edu>