TCPOLEN_MPTCP_MPC_SYNACK
len = TCPOLEN_MPTCP_MPC_SYNACK;
expected_opsize = TCPOLEN_MPTCP_MPC_SYNACK;
*size = TCPOLEN_MPTCP_MPC_SYNACK;
if (opsize >= TCPOLEN_MPTCP_MPC_SYNACK) {