Staging: batman-adv: best gw DHCP filter 802.1Q support

The DHCP filter inspects packets to determine whether or not to send
them via ethernet unicast. This patch adds 802.1Q (vlan) support for
this check.

Signed-off-by: Marek Lindner <lindner_marek@yahoo.de>
[sven.eckelmann@gmx.de: Rework on top of current version]
Signed-off-by: Sven Eckelmann <sven.eckelmann@gmx.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>