batman-adv: don't add TEMP clients belonging to other backbone nodes

The "early client detection" mechanism must not add clients belonging
to other backbone nodes. Such clients must be reached by directly
using the LAN instead of the mesh.

This was introduced by 30cfd02b


("batman-adv: detect not yet announced clients")

Reported-by: Glen Page <glen.page@thet.net>
Signed-off-by: Antonio Quartulli <ordex@autistici.org>
Signed-off-by: Marek Lindner <lindner_marek@yahoo.de>