NFS: Check return value from rpc_queue_upcall()

This function could fail to queue the upcall if rpc.idmapd is not running,
causing a warning message to be printed.  Instead, I want to check the
return value and revoke the key if the upcall can't be run.

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