blk-mq: fix regression from commit 624dbe4

When the code was collapsed to avoid duplication, the recent patch
for ensuring that a queue is idled before free was dropped, which was
added by commit 19c5d84.

Add back the blk_mq_tag_idle(), to ensure we don't leak a reference
to an active queue when it is freed.

Signed-off-by: Jens Axboe <axboe@fb.com>

block/blk-mq.c

@@ -1552,6 +1552,8 @@ static void blk_mq_exit_hw_queues(struct request_queue *q,
 		if (i == nr_queue)
 			break;
 
+		blk_mq_tag_idle(hctx);
+
 		if (set->ops->exit_hctx)
 			set->ops->exit_hctx(hctx, i);