UBIFS: fix replay regression

Commit "d51f17ea UBIFS: simplify reply code a bit" introduces a bug with the
following symptoms:

UBIFS error (pid 1): replay_log_leb: first CS node at LEB 3:0 has wrong commit number 0 expected 1

The issue is that we start replaying the log from UBIFS_LOG_LNUM instead
of c->lhead_lnum. This patch fixes that.

Reported-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>