block: unifying elevator change

Elevator change is one well-define behavior:

- tear down current elevator if it exists

- setup new elevator

It is supposed to cover any case for changing elevator by single
internal API, typically the following cases:

- setup default elevator in add_disk()

- switch to none in del_disk()

- reset elevator in blk_mq_update_nr_hw_queues()

- switch elevator in sysfs `store` elevator attribute

This patch uses elevator_change() to cover all above cases:

- every elevator switch is serialized with each other: add_disk/del_disk/
store elevator is serialized already, blk_mq_update_nr_hw_queues() uses
srcu for syncing with the other three cases

- for both add_disk()/del_disk(), queue freeze works at atomic mode
or has been froze, so the freeze in elevator_change() won't add extra
delay

- `struct elev_change_ctx` instance holds any info for changing elevator

Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Nilay Shroff <nilay@linux.ibm.com>
Reviewed-by: Hannes Reinecke <hare@suse.de>
Signed-off-by: Ming Lei <ming.lei@redhat.com>
Link: https://lore.kernel.org/r/20250505141805.2751237-17-ming.lei@redhat.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>

block/blk.h

@@ -323,9 +323,8 @@ bool blk_bio_list_merge(struct request_queue *q, struct list_head *list,
 bool blk_insert_flush(struct request *rq);
 
 void elv_update_nr_hw_queues(struct request_queue *q);
-void elevator_exit(struct request_queue *q);
-int elv_register_queue(struct request_queue *q, bool uevent);
-void elv_unregister_queue(struct request_queue *q);
+void elevator_set_default(struct request_queue *q);
+void elevator_set_none(struct request_queue *q);
 
 ssize_t part_size_show(struct device *dev, struct device_attribute *attr,
 		char *buf);