libceph: switch ceph_calc_pg_acting() to new helpers

Switch ceph_calc_pg_acting() to new helpers: pg_to_raw_osds(),
raw_to_up_osds() and apply_temps().

Signed-off-by: Ilya Dryomov <ilya.dryomov@inktank.com>
Reviewed-by: Alex Elder <elder@linaro.org>