Staging: batman-adv: Fix aggregation direct-link bug

So far, neighbour's secondary interface OGMs can involuntarily
piggyback on primary interface OGMs that arrived on the same secondary
interface before. Secondary interface OGMs should NEVER leave their
direct neighbour broadcast domain! This patch ensures that secondary
interface OGMs can only be aggregated to other secondary interface OGMs.

Signed-off-by: Linus Lüssing <linus.luessing@web.de>
Signed-off-by: Marek Lindner <lindner_marek@yahoo.de>
Signed-off-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>