rbd: delete an unnecessary check before rbd_dev_destroy()

The rbd_dev_destroy() function tests whether its argument is NULL
and then returns immediately. Thus the test around the call is not needed.

This issue was detected by using the Coccinelle software.
Signed-off-by: Markus Elfring <elfring@users.sourceforge.net>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>

drivers/block/rbd.c

@@ -5185,7 +5185,6 @@ static int rbd_dev_probe_parent(struct rbd_device *rbd_dev, int depth)
 
 out_err:
 	rbd_dev_unparent(rbd_dev);
-	if (parent)
 	rbd_dev_destroy(parent);
 	return ret;
 }