ipv6: enable anycast addresses as source addresses in ICMPv6 error messages

- Uses ipv6_anycast_destination() in icmp6_send().

Suggested-by: Bill Fink <billfink@mindspring.com>
Signed-off-by: Francois-Xavier Le Bail <fx.lebail@yahoo.com>
Acked-by: Hannes Frederic Sowa <hannes@stressinduktion.org>
Signed-off-by: David S. Miller <davem@davemloft.net>