CVE-2026-31683: batman-adv: avoid OGM aggregation when skb tailroom is insufficient

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

batman-adv: avoid OGM aggregation when skb tailroom is insufficient

When OGM aggregation state is toggled at runtime, an existing forwarded packet may have been allocated with only packetlen bytes, while a later packet can still be selected for aggregation. Appending in this case can hit skbput overflow conditions.

Reject aggregation when the target skb tailroom cannot accommodate the new packet. The caller then falls back to creating a new forward packet instead of appending.