CVE-2026-72232: batman-adv: ensure minimal ethernet header on TX

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

batman-adv: ensure minimal ethernet header on TX

As documented in commit 8bd67ebb50c0 ("net: bridge: xmit: make sure we have at least eth header len bytes"), it is possible by for a local user with eBPF TC hook access to attach a tc filter which truncates the packet and redirects to an batadv interface. But the code assumes that at least ETHHLEN bytes are available and thus might read outside of the available buffer.

The batadvinterfacetx() must therefore always check itself if enough data is available for the ethernet header and don't rely on minheaderlen.