nfsd: make nfs4_client->cl_addr a struct sockaddr_storage

It's currently a __be32, which isn't big enough to hold an IPv6 address.

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