batman-adv: do not add loop detection mac addresses to global tt

This change has been made for local TT already, add another one for
global TT - but only for temporary entries (aka speedy join), to prevent
inconsistencies between local and global tables in case an older
batman-adv version is still announcing those entries from its local
table.

Signed-off-by: Simon Wunderlich <sw@simonwunderlich.de>
Signed-off-by: Sven Eckelmann <sven@narfation.org>