CVE-2026-72417: netfilter: flowtable: Validate iph->ihl in nf_flow_ip4_tunnel_proto()

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

netfilter: flowtable: Validate iph->ihl in nfflowip4tunnelproto()

Add sanity check for iph->ihl field in nfflowip4tunnelproto() before using it to compute the header size, avoiding out-of-bounds access with malformed IP headers. While at it, use iph->protocol instead of the hardcoded IPPROTOIPIP constant when setting ctx->tun.proto and reference ctx->tun.hdrsize when updating ctx->offset.