IPv6: add the option to use anycast addresses as source addresses in echo reply

This change allows to follow a recommandation of RFC4942.

- Add "anycast_src_echo_reply" sysctl to control the use of anycast addresses
  as source addresses for ICMPv6 echo reply. This sysctl is false by default
  to preserve existing behavior.
- Add inline check ipv6_anycast_destination().
- Use them in icmpv6_echo_reply().

Reference:
RFC4942 - IPv6 Transition/Coexistence Security Considerations
   (http://tools.ietf.org/html/rfc4942#section-2.1.6

)

2.1.6. Anycast Traffic Identification and Security

   [...]
   To avoid exposing knowledge about the internal structure of the
   network, it is recommended that anycast servers now take advantage of
   the ability to return responses with the anycast address as the
   source address if possible.

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>