[PATCH v2 2/4] block: factor block size and alignment handling out of blk_stack_limits

[Yao Sang]

Thanks for the review.

The intended boundary is the mapped-range geometry: block sizes and
alignment for a top-device range starting at @start on the bottom device.
The max sector limits stay with it because they must be rounded after the
resulting logical block size is known. I'll clarify this in v3.

Thanks,
Yao