UBIFS: simplify reply code a bit

In the log reply code we assume that 'c->lhead_offs' is known and may be
non-zero, which is not the case because we do not store it in the master
node and have to find out by scanning on every mount. Knowing this fact
allows us to simplify the log scanning loop a bit and remove a couple
of unneeded local variables.

Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@linux.intel.com>