kernel: ip_vti: fix potential slab-use-after-free in decode_session6

A use-after-free flaw was found in the Linux kernel's ipvti IPsec Virtual Tunnel Interface implementation when transmitting IPv6 packets with the SFB qdisc attached. A local user with CAPNETADMIN capability can trigger this issue by configuring an ipvti interface with an SFB qdisc and sending IPv...