block: fold elevator_disable into elevator_switch

This removes duplicate code, and keeps the callers tidy.

Reviewed-by: Nilay Shroff <nilay@linux.ibm.com>
Reviewed-by: Hannes Reinecke <hare@suse.de>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Ming Lei <ming.lei@redhat.com>
Link: https://lore.kernel.org/r/20250505141805.2751237-12-ming.lei@redhat.com


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