NFS/knfsd: Remove the invalid NFS error 'NFSERR_OPNOTSUPP'

NFSERR_OPNOTSUPP is not described by any RFC, and should not be used.

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