CVE-2026-72227: batman-adv: mcast: avoid OOB read of num_dests header

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

batman-adv: mcast: avoid OOB read of numdests header

Before the access to struct batadvtvlvmcasttracker's numdests, it is attempted to check whether enough space is actually in the network header. But instead of using offsetofend() to check for the whole size (2) which must be accessible, offsetof() of is called. The latter is always returning 0. The comparison with the network header length will always return that enough data is available - even when only 1 or 0 bytes are accessible.

Instead of using offsetofend(), use the more common check for the whole header.