blk-mq: fix tags leak when shrink nr_hw_queues

Although we don't need to realloc set->tags[] when shrink nr_hw_queues,
we need to free them. Or these tags will be leaked.

How to reproduce:
1. mount -t configfs configfs /mnt
2. modprobe null_blk nr_devices=0 submit_queues=8
3. mkdir /mnt/nullb/nullb0
4. echo 1 > /mnt/nullb/nullb0/power
5. echo 4 > /mnt/nullb/nullb0/submit_queues
6. rmdir /mnt/nullb/nullb0

In step 4, will alloc 9 tags (8 submit queues and 1 poll queue), then
in step 5, new_nr_hw_queues = 5 (4 submit queues and 1 poll queue).
At last in step 6, only these 5 tags are freed, the other 4 tags leaked.

Signed-off-by: Chengming Zhou <zhouchengming@bytedance.com>
Reviewed-by: Ming Lei <ming.lei@redhat.com>
Link: https://lore.kernel.org/r/20230821095602.70742-1-chengming.zhou@linux.dev
Signed-off-by: Jens Axboe <axboe@kernel.dk>

Author: Chengming Zhou

block/blk-mq.c

@@ -4379,9 +4379,13 @@ static int blk_mq_realloc_tag_set_tags(struct blk_mq_tag_set *set,
 				       int new_nr_hw_queues)
 {
 	struct blk_mq_tags **new_tags;
+	int i;
 
-	if (set->nr_hw_queues >= new_nr_hw_queues)
+	if (set->nr_hw_queues >= new_nr_hw_queues) {
+		for (i = new_nr_hw_queues; i < set->nr_hw_queues; i++)
+			__blk_mq_free_map_and_rqs(set, i);
 		goto done;
+	}
 
 	new_tags = kcalloc_node(new_nr_hw_queues, sizeof(struct blk_mq_tags *),
 				GFP_KERNEL, set->numa_node);