block: factor out a chunk_size_left helper

Factor out a helper from blk_max_size_offset so that it can be reused
independently.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Bart Van Assche <bvanassche@acm.org>
Reviewed-by: Pankaj Raghav <p.raghav@samsung.com>
Link: https://lore.kernel.org/r/20220614090934.570632-2-hch@lst.de
Signed-off-by: Jens Axboe <axboe@kernel.dk>