[NETNS][IPV6] anycast - handle several network namespace

Make use of the network namespace information to have this protocol to
handle several network namespace.

Signed-off-by: Daniel Lezcano <dlezcano@fr.ibm.com>
Signed-off-by: Benjamin Thery <benjamin.thery@bull.net>
Signed-off-by: David S. Miller <davem@davemloft.net>