This mark prevents a multicast packet being flooded through the whole
mesh. The advantage of marking certain multicast packets via e.g.
ebtables instead of dropping is then the following:

This allows an administrator to let specific multicast packets pass as
long as they are forwarded to a limited number of nodes only and are
therefore creating no burdon to unrelated nodes.

Signed-off-by: Linus Lüssing <linus.luessing@c0d3.blue>