nfsd4: fix test_stateid error reply encoding

commit a11fcce1544df08c723d950ff0edef3adac40405 upstream.

If the entire operation fails then there's nothing to encode.

Signed-off-by: J. Bruce Fields <bfields@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>