NFSD: return nfsv4 error code nfserr_notsupp rather than nfsv[23]'s nfserr_opnotsupp

Thanks for Bill Baker at sun.com for catching this
at Connectathon 2009.

This bug was introduced in 2.6.27

Signed-off-by: Benny Halevy <bhalevy@panasas.com>
Signed-off-by: J. Bruce Fields <bfields@citi.umich.edu>