NFS: Change nfs_get_client() to take sockaddr *

Adjust arguments and callers of nfs_get_client() to pass a
"struct sockaddr *" instead of "struct sockaddr_in *" to support
non-IPv4 addresses.

Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Cc: Aurelien Charbon <aurelien.charbon@ext.bull.net>
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>