[Bridge] bride: IPv6 multicast snooping enhancements

[Linus Lüssing]

Hi,

Here are two, small feature changes I would like to submit to increase
the usefulness of the multicast snooping of the bridge code.

The first patch is an unaltered one I had submitted before, but since it
got no feedback I'm resubmitting it here for net-next. With the recently
added patch to disable snooping if there is no querier (b00589af + 248ba8ec05
+ 8d50af4fb), it should be a safe choice now (without these, patch 1/2 would
have introduced another potential for lost IPv6 multicast packets).

Both conceptually and also with some testing and fuzzing, I couldn't spot
any more causes for potential packet loss. And since the multicast snooping
code has now been tried by various people, I think it should be a safe
choice to apply the multicast snooping not only for IPv6 multicast packets
with a scope greater than link-local, but also for packets of exactly this
scope. The IPv6 standard mandates MLD reports for link-local multicast, too,
so we can safely snoop them as well (in contrast to IPv4 link-local).

Cheers, Linus