kernel: ip6_vti: fix slab-use-after-free in decode_session6

A use-after-free vulnerability was found in the IPv6 VTI Virtual Tunnel Interface implementation in the Linux kernel. When an IPv6 VTI device uses the SFB Stochastic Fair Blue qdisc, the control block cb field of an skb can be modified during packet enqueuing. The decodesession6 function then rea...