UBI: fix memory leak in update path

When truncating an UBI volume, UBI should allocates a PEB-sized
buffer but does not release it, which leads to memory leaks.
This patch fixes the issue.
Reported-by: NMarek Skuczynski <mareksk7@gmail.com>
Signed-off-by: NArtem Bityutskiy <Artem.Bityutskiy@nokia.com>
Tested-by: NMarek Skuczynski <mareksk7@gmail.com>
Cc: stable@kernel.org

drivers/mtd/ubi/upd.c

@@ -155,6 +155,7 @@ int ubi_start_update(struct ubi_device *ubi, struct ubi_volume *vol,
 		if (err)
 			return err;
 		vol->updating = 0;
+		return 0;
 	}
 
 	vol->upd_buf = vmalloc(ubi->leb_size);