batman-adv: mcast: shrink tracker packet after scrubbing

Remove all zero MAC address entries (00:00:00:00:00:00) from a multicast
packet's tracker TVLV before transmitting it and update all headers
accordingly. This way, instead of keeping the multicast packet at a
constant size throughout its journey through the mesh, it will become
more lightweight, smaller with every interested receiver on the way and
on each splitting intersection. Which can save some valuable bandwidth.

Signed-off-by: Linus Lüssing <linus.luessing@c0d3.blue>
Signed-off-by: Simon Wunderlich <sw@simonwunderlich.de>