CVE-2026-72319: ipvs: ensure inner headers in ICMP errors are in headroom

In the Linux kernel, the following vulnerability has been resolved:

ipvs: ensure inner headers in ICMP errors are in headroom

Sashiko points out that after stripping the outer headers with pskbpull() we should ensure the inner IP headers in ICMP errors from tunnels are present in the skb headroom for functions like ipv4updatepmtu(), icmpsend() and IPVSDBG().

Also, add more checks for the length of the inner headers.