AZL-90821

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

batman-adv: fix fragment reassembly length accounting

batman-adv keeps a running payload length for queued fragments and uses it to validate a fragment chain before reassembly.

That accounting currently allows the accumulated fragment length to be truncated during updates. As a result, malformed fragment chains can bypass the intended validation and drive reassembly with inconsistent length state, leading to a local denial of service.

Fix the accounting by storing the accumulated length in a length-typed field and rejecting update overflows before the existing validation logic runs.

The fix was verified against the original reproducer and against valid fragment reassembly paths.