libceph: rename ceph_osd_request::r_{oloc,oid} to r_base_{oloc,oid}

Rename ceph_osd_request::r_{oloc,oid} to r_base_{oloc,oid} before
introducing r_target_{oloc,oid} needed for redirects.

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