lockd: Enable NLM use of AF_INET6

If the kernel is configured to support IPv6 and the RPC server can register
services via rpcbindv4, we are all set to enable IPv6 support for lockd.

Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Cc: Aime Le Rouzic <aime.le-rouzic@bull.net>
Signed-off-by: J. Bruce Fields <bfields@citi.umich.edu>