ubifs: Fix typo of output in get_cs_sqnum

"Not a CS node" makes more sense than "Node a CS node".

Signed-off-by: Liu Song <liu.song11@zte.com.cn>
Reviewed-by: Jiang Biao <jiang.biao2@zte.com.cn>
Signed-off-by: Richard Weinberger <richard@nod.at>