block: null_blk: fix queue leak inside removing device

When queue_mode is NULL_Q_MQ and null_blk is being removed,
blk_cleanup_queue() isn't called to cleanup queue, so the queue
allocated won't be freed.

This patch calls blk_cleanup_queue() for MQ to drain all pending
requests first and release the reference counter of queue kobject, then
blk_mq_free_queue() will be called in queue kobject's release handler
when queue kobject's reference counter drops to zero.
Signed-off-by: Ming Lei <tom.leiming@gmail.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

drivers/block/null_blk.c

@@ -425,9 +425,6 @@ static void null_del_dev(struct nullb *nullb)
 	list_del_init(&nullb->list);
 
 	del_gendisk(nullb->disk);
-	if (queue_mode == NULL_Q_MQ)
-		blk_mq_free_queue(nullb->q);
-	else
 	blk_cleanup_queue(nullb->q);
 	put_disk(nullb->disk);
 	kfree(nullb);
@@ -578,9 +575,6 @@ static int null_add_dev(void)
 	disk = nullb->disk = alloc_disk_node(1, home_node);
 	if (!disk) {
 queue_fail:
-		if (queue_mode == NULL_Q_MQ)
-			blk_mq_free_queue(nullb->q);
-		else
 		blk_cleanup_queue(nullb->q);
 		cleanup_queues(nullb);
 err: