batman-adv: mcast: fix memory leak on deleting a batman-adv interface

The batman-adv multicast tracker TVLV handler is registered for the
new batman-adv multicast packet type upon creating a batman-adv interface,
but not unregistered again upon the interface's deletion, leading to a
memory leak.

Fix this memory leak by calling the according TVLV handler unregister
routine for the multicast tracker TVLV upon batman-adv interface
deletion.

Fixes: 07afe1ba ("batman-adv: mcast: implement multicast packet reception and forwarding")
Reported-and-tested-by:  <syzbot+ebe64cc5950868e77358@syzkaller.appspotmail.com>
Closes: https://lore.kernel.org/all/000000000000beadc4060f0cbc23@google.com/


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>