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: Marek Skuczynski <[e-mail]>
Signed-off-by: Artem Bityutskiy <[e-mail]>
Tested-by: Marek Skuczynski <[e-mail]>
Cc: [e-mail]

drivers/mtd/ubi/upd.c

···
 		if (err)
 			return err;
 		vol->updating = 0;
+		return 0;
 	}
 
 	vol->upd_buf = vmalloc(ubi->leb_size);