block: don't call freeze queue in elevator_switch() and elevator_disable()

block: don't call freeze queue in elevator_switch() and elevator_disable()

Both elevator_switch() and elevator_disable() are only called from the
two code paths, in which queue is guaranteed to be frozen.

So don't call freeze queue in the two functions, also add asserts for
queue freeze.

Reviewed-by: Christoph Hellwig <hch@lst.de>
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: Yu Kuai <yukuai3@huawei.com>
Link: https://lore.kernel.org/r/20250505141805.2751237-4-ming.lei@redhat.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>