bridge: apply multicast snooping to IPv6 link-local, too

The multicast snooping code should have matured enough to be safely
applicable to IPv6 link-local multicast addresses (excluding the
link-local all nodes address, ff02::1), too.

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