CVE-2026-72235: batman-adv: retrieve ethhdr after potential skb realloc on RX

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

batman-adv: retrieve ethhdr after potential skb realloc on RX

pskbmaypull() in batadvinterfacerx() could reallocate the buffer behind the skb. Variables which were pointing to the old buffer need to be reassigned to avoid an use-after-free.

This was done correctly for the VLAN header but missed for the ethernet header which is later used for the TT and AP isolation handling.