block: don't allow to switch elevator if updating nr_hw_queues is in-progress

Elevator switch code is another `nr_hw_queue` reader in non-fast-IO code
path, so it can't be done if updating `nr_hw_queues` is in-progress.

Take same approach with not allowing add/del disk when updating
nr_hw_queues is in-progress, by grabbing read lock of
set->update_nr_hwq_sema.

Reviewed-by: Hannes Reinecke <hare@suse.de>
Link: https://lore.kernel.org/linux-block/aAWv3NPtNIKKvJZc@fedora/ [1]
Reported-by: Shinichiro Kawasaki <shinichiro.kawasaki@wdc.com>
Closes: https://lore.kernel.org/linux-block/mz4t4tlwiqjijw3zvqnjb7ovvvaegkqganegmmlc567tt5xj67@xal5ro544cnc/
Signed-off-by: Ming Lei <ming.lei@redhat.com>
Reviewed-by: Nilay Shroff <nilay@linux.ibm.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Link: https://lore.kernel.org/r/20250505141805.2751237-10-ming.lei@redhat.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>

Author: Ming Lei

block/elevator.c

@@ -723,6 +723,7 @@ ssize_t elv_iosched_store(struct gendisk *disk, const char *buf,
 	int ret;
 	unsigned int memflags;
 	struct request_queue *q = disk->queue;
+	struct blk_mq_tag_set *set = q->tag_set;
 
 	/*
 	 * If the attribute needs to load a module, do it before freezing the
@@ -734,13 +735,15 @@ ssize_t elv_iosched_store(struct gendisk *disk, const char *buf,
 
 	elv_iosched_load_module(name);
 
+	down_read(&set->update_nr_hwq_lock);
 	memflags = blk_mq_freeze_queue(q);
 	mutex_lock(&q->elevator_lock);
 	ret = elevator_change(q, name);
 	if (!ret)
 		ret = count;
 	mutex_unlock(&q->elevator_lock);
 	blk_mq_unfreeze_queue(q, memflags);
+	up_read(&set->update_nr_hwq_lock);
 	return ret;
 }