block: introduce bdev_zone_start()

block: introduce bdev_zone_start()

Introduce the function bdev_zone_start() as a more explicit (and clear)
replacement for ALIGN_DOWN() to get the start sector of a zone
containing a particular sector of a zoned block device.

Use this new helper in blkdev_get_zone_info() and
blkdev_report_zones_cached().

Signed-off-by: Damien Le Moal <dlemoal@kernel.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Bart Van Assche <bvanassche@acm.org>
Reviewed-by: Hannes Reinecke <hare@suse.de>
Signed-off-by: Jens Axboe <axboe@kernel.dk>