block: get wp_offset by bdev_offset_from_zone_start

Call bdev_offset_from_zone_start() instead of open-coding it.

Fixes: dd291d77 ("block: Introduce zone write plugging")
Signed-off-by: LongPing Wei <weilongping@oppo.com>
Reviewed-by: Damien Le Moal <dlemoal@kernel.org>
Reviewed-by: Bart Van Assche <bvanassche@acm.org>
Link: https://lore.kernel.org/r/20241107020439.1644577-1-weilongping@oppo.com


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