batman-adv: mcast: apply optimizations for routable packets, too

Now that we not only track the presence of multicast listeners but also
multicast routers we can safely apply group-aware multicast-to-unicast
forwarding to packets with a destination address of scope greater than
link-local as well.
Signed-off-by: Linus Lüssing <linus.luessing@c0d3.blue>
Signed-off-by: Sven Eckelmann <sven@narfation.org>
Signed-off-by: Simon Wunderlich <sw@simonwunderlich.de>