NFSD: use nfserr for status after decode_cb_op_status

Bugs introduced in 85a56480
"NFSD: Update XDR decoders in NFSv4 callback client"

Cc: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: Benny Halevy <bhalevy@panasas.com>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>