NFS: Improve legacy idmapping fallback

Fallback should happen only when the request_key() call fails, because
this indicates that there was a problem running the nfsidmap program.
We shouldn't call the legacy code if the error was elsewhere.

Signed-off-by: Bryan Schumaker <bjschuma@netappp.com>
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>