block: cleanup variable naming in get_max_io_size

get_max_io_size has a very odd choice of variables names and
initialization patterns.  Switch to more descriptive names and more
clear initialization of them.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Link: https://lore.kernel.org/r/20220614090934.570632-5-hch@lst.de
Signed-off-by: Jens Axboe <axboe@kernel.dk>