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