block/mq-deadline: Fix a bug in deadline_from_pos()

A bug was introduced in deadline_from_pos() while implementing the
suggestion to use round_down() in the following code:

	pos -= bdev_offset_from_zone_start(rq->q->disk->part0, pos);

This patch makes deadline_from_pos() use round_down() such that 'pos' is
rounded down.

Reported-by: Shin'ichiro Kawasaki <shinichiro.kawasaki@wdc.com>
Closes: https://lore.kernel.org/all/5zthzi3lppvcdp4nemum6qck4gpqbdhvgy4k3qwguhgzxc4quj@amulvgycq67h/


Cc: Christoph Hellwig <hch@lst.de>
Cc: Damien Le Moal <dlemoal@kernel.org>
Fixes: 0effb390 ("block: mq-deadline: Handle requeued requests correctly")
Signed-off-by: Bart Van Assche <bvanassche@acm.org>
Link: https://lore.kernel.org/r/20230712173344.2994513-1-bvanassche@acm.org


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