UBI: Add fastmap stuff to attach.c

- Export compare_lebs() as fastmap needs this function.
- Implement fastmap scan logic.

Signed-off-by: Richard Weinberger <richard@nod.at>
Signed-off-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>

diff --cc drivers/mtd/ubi/attach.c
Simple merge

diff --cc drivers/mtd/ubi/build.c
Simple merge

diff --cc drivers/mtd/ubi/ubi.h
Simple merge