rbd: fix bug in rbd_dev_id_put()

In rbd_dev_id_put(), there's a loop that's intended to determine
the maximum device id in use.  But it isn't doing that at all,
the effect of how it's written is to simply use the just-put id
number, which ignores whole purpose of this function.

Fix the bug.
Signed-off-by: Alex Elder <elder@inktank.com>
Reviewed-by: Josh Durgin <josh.durgin@inktank.com>

drivers/block/rbd.c

@@ -2680,8 +2680,8 @@ static void rbd_dev_id_put(struct rbd_device *rbd_dev)
 		struct rbd_device *rbd_dev;
 
 		rbd_dev = list_entry(tmp, struct rbd_device, node);
-		if (rbd_id > max_id)
-			max_id = rbd_id;
+		if (rbd_dev->dev_id > max_id)
+			max_id = rbd_dev->dev_id;
 	}
 	spin_unlock(&rbd_dev_list_lock);