librbd: fix diff_iterate arithmetic for non-standard striping

This code is confusing because we are moving back and forth between
image offsets, "buffer" offsets (image offsets relative to off), and
object offsets.  Fix the math.

Signed-off-by: Sage Weil <sage@inktank.com>
qa: rbd/diff_continuous.sh: use non-standard striping

Exercise the striping arithmetic by using non-standard striping that
varies between the parent and child.

Signed-off-by: Sage Weil <sage@inktank.com>