block: add `struct elv_change_ctx` for unifying elevator change

Add `struct elv_change_ctx` and prepare for unifying elevator change by
elevator_change(). With this way, any input & output parameter can
be provided & observed in top helper.

This way helps to move kobject add/delete & debugfs register/unregister
out of ->elevator_lock & freezing queue.

Reviewed-by: Hannes Reinecke <hare@suse.de>
Reviewed-by: Nilay Shroff <nilay@linux.ibm.com>
Signed-off-by: Ming Lei <ming.lei@redhat.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Link: https://lore.kernel.org/r/20250505141805.2751237-16-ming.lei@redhat.com


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