nfs: standardize the rename response container

Right now, v3 and v4 have their own variants. Create a standard struct
that will work for v3 and v4. v2 doesn't get anything but a simple error
and so isn't affected by this.

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