batman-adv: Reject unicast packet with zero/mcast dst address

An unicast batman-adv packet cannot be transmitted to a multicast or zero
mac address. So reject incoming packets which still have these classes of
addresses as destination mac address in the outer ethernet header.

Signed-off-by: Sven Eckelmann <sven@narfation.org>
Signed-off-by: Simon Wunderlich <sw@simonwunderlich.de>