UBI: incorporate LEB offset information

Incorporate the LEB offset information into UBIFS. We'll use this
information in one of the next patches to figure out what are the
max. write size offsets relative to the PEB. So this patch is just
a preparation.

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

diff --cc fs/ubifs/super.c
Simple merge

diff --cc fs/ubifs/ubifs.h
Simple merge