NFSv4: Allow attribute caching with 'noac' mounts if client holds a delegation

If the server has given us a delegation on a file, we _know_ that we can
cache the attribute information even when the user has specified 'noac'.

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