ceph: fix object striping calculation for non-default striping schemes

We were incorrectly calculationing of object offset.  If we have multiple
stripe units per object, we need to shift to the start of the current
su in addition to the offset within the su.

Also rename bno to ono (object number) to avoid some variable naming
confusion.

Signed-off-by: Sage Weil <sage@newdream.net>