ore: cleanup: Embed an ore_striping_info inside ore_io_state

Now that each ore_io_state covers only a single raid group.
A single striping_info math is needed. Embed one inside
ore_io_state to cache the calculation results and eliminate
an extra call.

Also the outer _prepare_for_striping is removed since it does nothing.

Signed-off-by: Boaz Harrosh <bharrosh@panasas.com>