ipv6: silence log messages for locally generated multicast

This patch fixes minor annoyance during transmission of unsolicited
neighbor advertisements from userspace to multicast addresses (as
far as I can see in RFC, this is allowed and the similar functionality
for IPv4 has been in arping for a long time).

Outgoing multicast packets get reinserted into local processing as if they
are received from the network. The machine thus sees its own NA and fills
the logs with error messages. This patch removes the message if NA has been
generated locally.

Signed-off-by: Jan Sembera <jsembera@suse.cz>
Signed-off-by: David S. Miller <davem@davemloft.net>