batman-adv: Always flood IGMP/MLD reports

With this patch IGMP or MLD reports are always flooded. This is
necessary for the upcoming bridge integration to function without
multicast packet loss.

With the report handling so far bridges might miss interested multicast
listeners, leading to wrongly excluding ports from multicast packet
forwarding.

Currently we are treating IGMP/MLD reports, the messages bridges use to
learn about interested multicast listeners, just as any other multicast
packet: We try to send them to nodes matching its multicast destination.

Unfortunately, the destination address of reports of the older
IGMPv2/MLDv1 protocol families do not strictly adhere to their own
protocol: More precisely, the interested receiver, an IGMPv2 or MLDv1
querier, itself usually does not listen to the multicast destination
address of any reports.

Therefore with this patch we are simply excluding IGMP/MLD reports from
the multicast forwarding code path and keep flooding them. By that
any bridge receives them and can properly learn about listeners.

To avoid compatibility issues with older nodes not yet implementing this
report handling, we need to force them to flood reports: We do this by
bumping the multicast TVLV version to 2, effectively disabling their
multicast optimization.

Tested-by: Simon Wunderlich <sw@simonwunderlich.de>
Signed-off-by: Linus Lüssing <linus.luessing@c0d3.blue>
Signed-off-by: Marek Lindner <mareklindner@neomailbox.ch>
Signed-off-by: Sven Eckelmann <sven@narfation.org>
Signed-off-by: Simon Wunderlich <sw@simonwunderlich.de>