ebtables actually skips any IPv6 extension headers like the hop-by-hop
one. So this rule is actually void.

The intend back then was to allow passing MLD messages into the mesh.
Since extension headers are skipped, the general icmpv6 rule will
actually match MLD messages. So the hop-by-hop rule is unnecessary,
too.

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

in a layer 2 mesh network, multicast pings cause a lot of traffic in the
network, significantly increasing the 'backgroudn noise' (= Grundrauschen)
and stressing nodes in the network.

this commit blacklists all icmpv4 multicast traffic as well as multicast
icmpv6 echo-requests and node iformation queries. as no application
depending on these types of multicast traffic is known, blacklisting is safe.

Fixes https://github.com/freifunk-gluon/gluon/issues/311

gluon-ebtables-filter-multicast: allow hop-by-hop options as ICMPv6 is sometimes prefixed by an HBH header

the ports were interchanged, see the following packet flow:

client:546 --> [ff02::1:2]:547
server:547 --> client:546

therefore we need to allow outgoing multicast packets with dst-port 547
and unicast packets from bat0 to clients with dst-port 546 and 547 in the other direction