bridge: Allow mcast snooping for transient link local addresses too

Currently the multicast bridge snooping support is not active for
link local multicast. I assume this has been done to leave
important multicast data untouched, like IPv6 Neighborhood Discovery.

In larger, bridged, local networks it could however be desirable to
optimize for instance local multicast audio/video streaming too.

With the transient flag in IPv6 multicast addresses we have an easy
way to optimize such multimedia traffic without tempering with the
high priority multicast data from well-known addresses.

This patch alters the multicast bridge snooping for IPv6, to take
effect for transient multicast addresses instead of non-link-local
addresses.

Signed-off-by: Linus Lüssing <linus.luessing@web.de>
Signed-off-by: David S. Miller <davem@davemloft.net>