mptcp: use proper req destructor for IPv6
This is an automated ID intended to aid in discovery of potential security vulnerabilities. The actual impact and attack plausibility have not yet been proven. This ID is fixed in Linux Kernel version v5.15.87 by commit bd5dc96fea4edd16d2e22f41b4dd50a4cfbeb919, it was introduced in version v5.6 by commit 79c0949e9a09f6a14a6dd18dc8396029423f9b68. For more details please see the references link.