NLM: Fix GRANT callback address comparison when IPv6 is enabled

The NFS mount command may pass an AF_INET server address to lockd.  If
lockd happens to be using a PF_INET6 listener, the nlm_cmp_addr() in
nlmclnt_grant() will fail to match requests from that host because they
will all have a mapped IPv4 AF_INET6 address.

Adopt the same solution used in nfs_sockaddr_match_ipaddr() for NFSv4
callbacks: if either address is AF_INET, map it to an AF_INET6 address
before doing the comparison.

Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>