block: introduce bdev_iter_is_aligned helper

Provide a convenient function for this repeatable coding pattern.

Signed-off-by: Keith Busch <kbusch@kernel.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Link: https://lore.kernel.org/r/20220610195830.3574005-10-kbusch@fb.com


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