UBI: fix ubi free PEBs count calculation

The ubi->free_count should be updated with every insert/remove to/from
the ubi->free list.
Signed-off-by: Tanya Brokhman <tlinder@codeaurora.org>
Reviewed-by: Dolev Raviv <draviv@codeaurora.org>
Acked-by: Richard Weinberger <richard@nod.at>
Signed-off-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>

drivers/mtd/ubi/wl.c

@@ -671,6 +671,8 @@ static struct ubi_wl_entry *get_peb_for_wl(struct ubi_device *ubi)
 
 	e = find_wl_entry(ubi, &ubi->free, WL_FREE_MAX_DIFF);
 	self_check_in_wl_tree(ubi, e, &ubi->free);
+	ubi->free_count--;
+	ubi_assert(ubi->free_count >= 0);
 	rb_erase(&e->u.rb, &ubi->free);
 
 	return e;
@@ -1071,6 +1073,7 @@ static int wear_leveling_worker(struct ubi_device *ubi, struct ubi_work *wrk,
 
 			/* Give the unused PEB back */
 			wl_tree_add(e2, &ubi->free);
+			ubi->free_count++;
 			goto out_cancel;
 		}
 		self_check_in_wl_tree(ubi, e1, &ubi->used);