OUT UPDATE Request always lists reply size as 4K in out_update_header (lustre/osp/osp_trans.c::osp_prep_update_req())

OUT UPDATE Reply blindly uses this size to create the pack size, not actually checking the size of the packed reply. lustre/target/out_handler.c::out_handle())

If the former behavior is changed, than the later behavior will break.  Currently there's a 4K buffer to hold ~100B of data.