libceph: CEPH_OSD_FLAG_* enum update

Update CEPH_OSD_FLAG_* enum.  (We need CEPH_OSD_FLAG_IGNORE_OVERLAY to
support tiering).

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