UBIFS: call dbg_is_power_cut() instead of reading c->dbg->pc_happened

Call dbg_is_power_cut() to emulate power cut instead of reading
c->dbg->pc_happened. Otherwise, the function becomes dead code.

Signed-off-by: Sheng Yong <shengyong1@huawei.com>
Signed-off-by: Richard Weinberger <richard@nod.at>