UBI: fix out of bounds write

commit d74adbdb 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>

drivers/mtd/ubi/eba.c

@@ -1261,7 +1261,8 @@ int ubi_eba_init_scan(struct ubi_device *ubi, struct ubi_scan_info *si)
 				 * during re-size.
 				 */
 				ubi_scan_move_to_list(sv, seb, &si->erase);
-			vol->eba_tbl[seb->lnum] = seb->pnum;
+			else
+				vol->eba_tbl[seb->lnum] = seb->pnum;
 		}
 	}