block: call rq_qos_exit() after queue is frozen

rq_qos_exit() removes the current q->rq_qos, this action has to be
done after queue is frozen, otherwise the IO queue path may never
be waken up, then IO hang is caused.

So fixes this issue by moving rq_qos_exit() after queue is frozen.

Cc: Josef Bacik <josef@toxicpanda.com>
Signed-off-by: NMing Lei <ming.lei@redhat.com>
Signed-off-by: NJens Axboe <axboe@kernel.dk>

block/blk-core.c

@@ -785,6 +785,9 @@ void blk_cleanup_queue(struct request_queue *q)
 	 * prevent that q->request_fn() gets invoked after draining finished.
 	 */
 	blk_freeze_queue(q);
+
+	rq_qos_exit(q);
+
 	spin_lock_irq(lock);
 	queue_flag_set(QUEUE_FLAG_DEAD, q);
 	spin_unlock_irq(lock);

block/blk-sysfs.c

@@ -1007,8 +1007,6 @@ void blk_unregister_queue(struct gendisk *disk)
 	kobject_del(&q->kobj);
 	blk_trace_remove_sysfs(disk_to_dev(disk));
 
-	rq_qos_exit(q);
-
 	mutex_lock(&q->sysfs_lock);
 	if (q->request_fn || (q->mq_ops && q->elevator))
 		elv_unregister_queue(q);