block: move elv_register[unregister]_queue out of elevator_lock

Move elv_register[unregister]_queue out of ->elevator_lock & queue freezing,
so we can kill many lockdep warnings.

elv_register[unregister]_queue() is serialized, and just dealing with sysfs/
debugfs things, no need to be done with queue frozen:

- when it is called from adding disk, elevator switch isn't possible
  because ->queue_kobj isn't added yet

- when it is called from deleting disk, disable_elv_switch() is
  responsible for preventing new elevator switch and draining old
  elevator switch.

- when it is called from blk_mq_update_nr_hw_queues(), adding/removing
  disk and elevator switch can't be allowed or in-progress

With this change, elevator's ->exit() is called before calling
elv_unregister_queue, then user may call into ->show()/store() of elevator's
sysfs attributes, and we have covered this issue by adding `ELEVATOR_FLAG_DYNG`.

For blk-mq debugfs, hctx->sched_tags is always checked with ->elevator_lock by
debugfs code, meantime hctx->sched_tags is updated with ->elevator_lock, so
there isn't such issue.

Reviewed-by: Hannes Reinecke <hare@suse.de>
Signed-off-by: Ming Lei <ming.lei@redhat.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Link: https://lore.kernel.org/r/20250505141805.2751237-22-ming.lei@redhat.com


Signed-off-by: Jens Axboe <axboe@kernel.dk>