mptcp_pm_is_backup
bool mptcp_pm_is_backup(struct mptcp_sock *msk, struct sock_common *skc);
subflow_req->request_bkup = mptcp_pm_is_backup(msk, (struct sock_common *)req);
subflow->request_bkup = mptcp_pm_is_backup(msk, (struct sock_common *)sk);