ubifs: Remove the redundant return in dbg_check_nondata_nodes_order

There is a redundant return in dbg_check_nondata_nodes_order,
which will be never reached. In addition, error code should be
returned instead of zero in this branch.

Signed-off-by: Chengsong Ke <kechengsong@huawei.com>
Signed-off-by: Richard Weinberger <richard@nod.at>