UBI: fix out of bounds write

commit d74adbdb9abf0d2506a6c4afa534d894f28b763f upstream.

If aeb->len >= vol->reserved_pebs, we should not be writing aeb into the
PEB->LEB mapping.

Caught by Coverity, CID #711212.

Signed-off-by: Brian Norris <computersforpeace@gmail.com>
Signed-off-by: Richard Weinberger <richard@nod.at>
[bwh: Backported to 3.2: adjust context; s/leb/seb/g]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>

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