mptcp_is_checksum_enabled
opts->csum_reqd = mptcp_is_checksum_enabled(sock_net(sk));
WRITE_ONCE(msk->csum_enabled, mptcp_is_checksum_enabled(sock_net(sk)));
WRITE_ONCE(msk->csum_enabled, mptcp_is_checksum_enabled(sock_net(sk)));
int mptcp_is_checksum_enabled(const struct net *net);
subflow_req->csum_reqd = mptcp_is_checksum_enabled(sock_net(sk_listener));