NFS: return -ENOKEY when the upcall fails to map the name

This allows the normal error-paths to handle the error, rather than
making a special call to complete_request_key() just for this instance.

Signed-off-by: Bryan Schumaker <bjschuma@netapp.com>
Tested-by: William Dauchy <wdauchy@gmail.com>
Cc: stable@vger.kernel.org [>= 3.4]
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>