* Rename _offset_dev_unit_off() to _calc_stripe_info()
and recieve a struct for the output params
* In _prepare_for_striping we only need to call
_calc_stripe_info() once. The other componets
are easy to calculate from that. This code
was inspired by what's done in truncate.
* Some code shifts that make sense now but will make
more sense when group support is added.
Signed-off-by: Boaz Harrosh <bharrosh@panasas.com>