UBI: Fastmap: Fix leb_count unbalance

If a LEB is unmapped we have to decrement leb_count as well.
Signed-off-by: Richard Weinberger <richard@nod.at>

drivers/mtd/ubi/fastmap.c

@@ -362,6 +362,7 @@ static void unmap_peb(struct ubi_attach_info *ai, int pnum)
 			aeb = rb_entry(node2, struct ubi_ainf_peb, u.rb);
 			if (aeb->pnum == pnum) {
 				rb_erase(&aeb->u.rb, &av->root);
+				av->leb_count--;
 				kmem_cache_free(ai->aeb_slab_cache, aeb);
 				return;
 			}