BACKPORT: FROMLIST: block: add a new helper bdev_{is_zone_start, offset_from_zone_start}

Instead of open coding to check for zone start, add a helper to improve
readability and store the logic in one place.

Reviewed-by: Chaitanya Kulkarni <kch@nvidia.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Pankaj Raghav <p.raghav@samsung.com>
Bug: 197782466
Bug: 269471019
Link: https://lore.kernel.org/linux-block/20230110143635.77300-3-p.raghav@samsung.com/


Change-Id: Ieb3ec909589fe087f49a61c48ba85f0e612b6d1d
Signed-off-by: Bart Van Assche <bvanassche@google.com>