batman-adv: mcast: collect softif listeners from IP lists instead

Instead of collecting multicast MAC addresses from the netdev hw mc
list collect a node's multicast listeners from the IP lists and convert
those to MAC addresses.

This allows to exclude addresses of specific scope later. On a
multicast MAC address the IP destination scope is not visible anymore.

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>