Staging: batman-adv: add gateway IPv6 support by filtering DHCPv6 messages

Some additional checks will be needed in case of extension headers
like the fragmentation or hop-by-hop (for jumbo frames for example)
headers or ipsec stuff. But this patch should do for most people
for now, the rest can be added with a later one.

Signed-off-by: Marek Lindner <lindner_marek@yahoo.de>
Acked-by: Linus Lüssing <linus.luessing@web.de>
Signed-off-by: Sven Eckelmann <sven.eckelmann@gmx.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>