block: Ensure start sector is aligned for stacking atomic writes

For stacking atomic writes, ensure that the start sector is aligned with
the device atomic write unit min and any boundary. Otherwise, we may
permit misaligned atomic writes.

Rework bdev_can_atomic_write() into a common helper to resuse the
alignment check. There also use atomic_write_hw_unit_min, which is more
proper (than atomic_write_unit_min).

Fixes: d7f36dc4 ("block: Support atomic writes limits for stacked devices")
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: John Garry <john.g.garry@oracle.com>
Reviewed-by: Martin K. Petersen <martin.petersen@oracle.com>
Link: https://lore.kernel.org/r/20250109114000.2299896-2-john.g.garry@oracle.com


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