rbd: fix double free on rbd_dev->header_name

If rbd_dev_image_probe() in rbd_dev_probe_parent() fails, header_name
is freed twice: once in rbd_dev_probe_parent() and then in its caller
rbd_dev_image_probe() (rbd_dev_image_probe() is called recursively to
handle parent images).

rbd_dev_probe_parent() is responsible for probing the parent, so it
shouldn't muck with clone's fields.
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>
Reviewed-by: Alex Elder <elder@linaro.org>

drivers/block/rbd.c

@@ -5201,7 +5201,6 @@ static int rbd_dev_probe_parent(struct rbd_device *rbd_dev)
 out_err:
 	if (parent) {
 		rbd_dev_unparent(rbd_dev);
-		kfree(rbd_dev->header_name);
 		rbd_dev_destroy(parent);
 	} else {
 		rbd_put_client(rbdc);