mptcp: use proper req destructor for IPv6

In the Linux kernel, the following vulnerability has been resolved: mptcp: use proper req destructor for IPv6 Before, only the destructor from TCP request sock in IPv4 was called even if the subflow was IPv6. It is important to use the right destructor to avoid memory leaks with some advanced IPv6 features, e.g. when the request socks contain specific IPv6 options.