Staging: batman-adv: always reply batman icmp packets with primary mac

When receiving an batman icmp echo request or in case of a time-to-live
exceeded batman would reply with the mac address of the outgoing
interface which might be a secondary interface. Because secondary
interfaces are not globally known this might lead to confusion.
Now, replies are sent with the mac address of the primary interface.

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