batman-adv: Drop tt queries with foreign dest

When enabling promiscuous mode, tt queries for other hosts might be
received. Before this patch, "foreign" tt queries were processed like
any other query and thus forwarded to its destination again and thereby
causing a loop.

This patch adds a check to drop foreign tt queries.

Signed-off-by: Martin Hundebøll <martin@hundeboll.net>
Signed-off-by: Antonio Quartulli <ordex@autistici.org>