xlog_grant_sub_space
xlog_grant_sub_space(&log->l_reserve_head, diff);
xlog_grant_sub_space(&log->l_write_head, diff);
xlog_grant_sub_space(&log->l_reserve_head, ticket->t_curr_res);
xlog_grant_sub_space(&log->l_write_head, ticket->t_curr_res);
xlog_grant_sub_space(&log->l_reserve_head, bytes);
xlog_grant_sub_space(&log->l_write_head, bytes);