batman-adv: filter ARP packets with invalid MAC addresses in DAT

We never want multicast MAC addresses in the Distributed ARP Table, so it's
best to completely ignore ARP packets containing them where we expect unicast
addresses.

Signed-off-by: Matthias Schiffer <mschiffer@universe-factory.net>
Acked-by: Antonio Quartulli <ordex@autistici.org>
Signed-off-by: Marek Lindner <lindner_marek@yahoo.de>
Signed-off-by: Antonio Quartulli <ordex@autistici.org>