tunnels: do not assume mac header is set in skbtunnelcheck_pmtu()
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 v5.15.53 by commit 674a641e5b67e16ba3112eacd680ff87b38539de, it was introduced in version v5.9 by commit 4cb47a8644cc9eb8ec81190a50e79e6530d0297f. For more details please see the references link.