CVE-2026-63992 tunnels: do not assume transport header in iptunnel_pmtud_check_icmp()

In the Linux kernel, the following vulnerability has been resolved: tunnels: do not assume transport header in iptunnelpmtudcheckicmp In some cases, iptunnelpmtudcheckicmp can be called while skb transport header is not set. This triggers an out-of-bound access, because typeofskb-transportheader0...