tunnels: do not assume mac header is set in skb_tunnel_check_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 v5.18.10 by commit 32dcf62efa0003f92a976aea0c57f118e689de8b, it was introduced in version v5.9 by commit 4cb47a8644cc9eb8ec81190a50e79e6530d0297f. For more details please see the references link.