SUNRPC: Don't display zero scope IDs

A zero scope ID means that it wasn't set, so we don't need to append
it to presentation format addresses.

Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Acked-by: Jeff Layton <jlayton@redhat.com>
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>