libceph: replace ceph_calc_ceph_pg() with ceph_oloc_oid_to_pg()

Switch ceph_calc_ceph_pg() to new oloc and oid abstractions and rename
it to ceph_oloc_oid_to_pg() to make its purpose more clear.

Signed-off-by: Ilya Dryomov <ilya.dryomov@inktank.com>
Reviewed-by: Sage Weil <sage@inktank.com>