SO_SNDTIMEO doesn't seem to take effect correctly on MPTCP sockets

[WOnder93]

How to reproduce:

1. Save the following as client.c:

#include <stdio.h>
#include <string.h>

#include <unistd.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <netdb.h>

int main(int argc, char **argv)
{
	int rc, sock;
	struct addrinfo hints, *res;
	struct timeval tm;

	memset(&hints, 0, sizeof(struct addrinfo));
	hints.ai_socktype = SOCK_STREAM;
	hints.ai_protocol = IPPROTO_TCP;

	rc = getaddrinfo("8.8.8.8", "9999", &hints, &res);
	if (rc < 0) {
		fprintf(stderr, "getaddrinfo: %s\n",
			gai_strerror(rc));
		return 1;
	}

	sock = socket(res->ai_family, res->ai_socktype, res->ai_protocol);
	if (sock < 0) {
		perror("socket");
		return 1;
	}

	tm.tv_sec = 2;
	tm.tv_usec = 0;
	rc = setsockopt(sock, SOL_SOCKET, SO_SNDTIMEO, &tm, sizeof(tm));
	if (rc < 0) {
		perror("setsockopt: SO_SNDTIMEO");
		return 1;
	}

	rc = connect(sock, res->ai_addr, res->ai_addrlen);
	if (rc < 0) {
		perror("connect");
		close(sock);
		return 1;
	}
	close(sock);
	return 0;
}

2. Compile it with gcc -o client client.c.
3. Run time mptcpize run ./client.

Expected result: connect(2) times out after about 2 seconds, which is the specified SO_SNDTIMEO value. With plain TCP it works as expected:

$ time ./client 
connect: Operation now in progress

real    0m2,017s
user    0m0,001s
sys     0m0,004s

Actual result:

$ time mptcpize run ./client 
connect: Connection timed out

real    2m9,989s
user    0m0,001s
sys     0m0,004s