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 pskb_may_pull() in batadv_interface_rx() 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.