inet6: prevent network storms caused by linux IPv6 routers

Linux IPv6 forwards unicast packets, which are link layer multicasts...
The hole was present since day one. I was 100% this check is there, but it is not.

The problem shows itself, f.e. when Microsoft Network Load Balancer runs on a network.
This software resolves IPv6 unicast addresses to multicast MAC addresses.

Signed-off-by: Alexey Kuznetsov <kuznet@ms2.inr.ac.ru>
Signed-off-by: David S. Miller <davem@davemloft.net>