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 v5.10.129 by commit 59c51c3b545128a92ebfb6dbae990d3abee110e7, it was introduced in version v5.9 by commit 4cb47a8644cc9eb8ec81190a50e79e6530d0297f. For more details please see the references link.