batman-adv: update to version 2026.3

batman-adv: update to version 2026.3

From upstream changelog
https://git.open-mesh.org/batman-adv.git/tree/CHANGELOG.rst:

* support latest kernels (5.15 - 7.3)
* coding style cleanups and refactoring
* reduce memory and runtime overhead by only handling attached interfaces
* optimize throughput meters unacked packets handling
* bugs squashed:
  - clean untagged VLAN on netdev registration failure
  - ensure minimal ethernet header on TX
  - fix VLAN priority offset
  - bla: avoid CRC corruption due to parallel claim add
  - bla: fix freeing of claims on meshif deletion
  - bla: prevent CRC corruptions after claim flush
  - dat: atomically update mac addresses
  - dat: avoid unaligned fault in IP extraction
  - dat: fix tie-break for candidate selection
  - frag: fix primary_if leak on failed linearization
  - frag: fix stale receive device on merged fragments
  - frag: free unfragmentable packet
  - mcast: avoid OOB read of num_dests header
  - mcast: fix TX priority extraction for BATADV_FORW_MCAST
  - mcast: ensure unshared skb for multicast packets
  - mcast: linearize skbuff for packet generation
  - mcast: reject unrepresentable TVLV offsets
  - tt: avoid request storms during pending request
  - tt: prevent TVLV OOB check overflow
  - tvlv: handle negative tvlv processing return codes

Signed-off-by: Sven Eckelmann <redacted>