xen-blkback: really don't leak mode property

Commit 9d092603 ("xen-blkback: do not leak mode property") left one
path unfixed; correct this.
Acked-by: Jens Axboe <axboe@kernel.dk>
Acked-by: Roger Pau Monné <roger.pau@citrix.com>
Signed-off-by: Jan Beulich <jbeulich@suse.com>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>

drivers/block/xen-blkback/xenbus.c

@@ -715,8 +715,11 @@ static void backend_changed(struct xenbus_watch *watch,
 
 	/* Front end dir is a number, which is used as the handle. */
 	err = kstrtoul(strrchr(dev->otherend, '/') + 1, 0, &handle);
-	if (err)
+	if (err) {
+		kfree(be->mode);
+		be->mode = NULL;
 		return;
+	}
 
 	be->major = major;
 	be->minor = minor;