CVE-2022-50816 ipv6: ensure sane device mtu in tunnels

In the Linux kernel, the following vulnerability has been resolved: ipv6: ensure sane device mtu in tunnels Another syzbot report 1 with no reproducer hints at a bug in ip6gre tunnel dev:ip6gretap0 Since ipv6 mcast code makes sure to read dev-mtu once and applies a sanity check on it see commit...