xlog_grant_add_space
xlog_grant_add_space(&log->l_reserve_head, roundoff);
xlog_grant_add_space(&log->l_write_head, roundoff);
xlog_grant_add_space(&log->l_reserve_head, ticket->t_unit_res);
xlog_grant_add_space(&log->l_write_head, need_bytes);
xlog_grant_add_space(&log->l_reserve_head, need_bytes);
xlog_grant_add_space(&log->l_write_head, need_bytes);