libceph: encode CEPH_OSD_OP_FLAG_* op flags

Encode ceph_osd_op::flags field so that it gets sent over the wire.

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