batman-adv: Disallow mcast src address for data frames

The routing checks are validating the source mac address of the outer
ethernet header. They reject every source mac address which is a broadcast
address. But they also have to reject any multicast mac addresses.

Signed-off-by: Sven Eckelmann <sven@narfation.org>
[sw@simonwunderlich.de: fix commit message typo]
Signed-off-by: Simon Wunderlich <sw@simonwunderlich.de>