batman-adv: make the Distributed ARP Table vlan aware

The same IP subnet can be used on different VLANs, therefore
DAT has to differentiate whether the IP to resolve belongs
to one or the other virtual LAN.
To accomplish this task DAT has to deal with the VLAN tag
and store it together with each ARP entry.

Signed-off-by: Antonio Quartulli <antonio@open-mesh.com>
Signed-off-by: Marek Lindner <lindner_marek@yahoo.de>