nfsd: only fill out return pointer on success in nfsd4_lookup_stateid

In the case of a revoked delegation, we still fill out the pointer even
when returning an error, which is bad form. Only overwrite the pointer
on success.

Signed-off-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>