block: fold blk_max_size_offset into get_max_io_size

Now that blk_max_size_offset has a single caller left, fold it into that
and clean up the naming convention for the local variables there.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Pankaj Raghav <p.raghav@samsung.com>
Reviewed-by: Bart Van Assche <bvanassche@acm.org>
Link: https://lore.kernel.org/r/20220614090934.570632-6-hch@lst.de
Signed-off-by: Jens Axboe <axboe@kernel.dk>