batman-adv: mcast/TT: fix wrongly dropped or rerouted packets

batman-adv: mcast/TT: fix wrongly dropped or rerouted packets

The unicast packet rerouting code makes several assumptions. For
instance it assumes that there is always exactly one destination in the
TT. This breaks for multicast frames in a unicast packets in several ways:

For one thing if there is actually no TT entry and the destination node
was selected due to the multicast tvlv flags it announced. Then an
intermediate node will wrongly drop the packet.

For another thing if there is a TT entry but the TTVN of this entry is
newer than the originally addressed destination node: Then the
intermediate node will wrongly redirect the packet, leading to
duplicated multicast packets at a multicast listener and missing
packets at other multicast listeners or multicast routers.

Fixing this by not applying the unicast packet rerouting to batman-adv
unicast packets with a multicast payload. We are not able to detect a
roaming multicast listener at the moment and will just continue to send
the multicast frame to both the new and old destination for a while in
case of such a roaming multicast listener.

Fixes: a73105b8d4c7 ("batman-adv: improved client announcement mechanism")
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>