knfsd: LOOKUP can return an illegal error value

The 'NFS error' NFSERR_OPNOTSUPP is not described by any of the official
NFS related RFCs, but appears to have snuck into some older .x files for
NFSv2.
Either way, it is not in RFC1094, RFC1813 or any of the NFSv4 RFCs, so
should not be returned by the knfsd server, and particularly not by the
"LOOKUP" operation.

Instead, let's return NFSERR_STALE, which is more appropriate if the
filesystem encodes the filehandle as FILEID_INVALID.

Cc: stable@vger.kernel.org
Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>