dm raid: fix random optimal_io_size for raid0

raid_io_hints() was retrieving the number of data stripes used for the
calculation of io_opt from struct r5conf, which is not defined for raid0
mappings.

Base the calculation on the in-core raid_set structure instead.

Also, adjust to use to_bytes() for the sector -> bytes conversion
throughout.

Signed-off-by: Heinz Mauelshagen <heinzm@redhat.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>