block: move blk_queue_registered() check into elv_iosched_store()

Move blk_queue_registered() check into elv_iosched_store() and prepare
for using elevator_change() for covering any kind of elevator change in
adding/deleting disk and updating nr_hw_queue.

Reviewed-by: Hannes Reinecke <hare@suse.de>
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/20250505141805.2751237-13-ming.lei@redhat.com


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