[NETNS][IPV6] addrconf - make addrconf per namespace

[NETNS][IPV6] addrconf - make addrconf per namespace

All the infrastructure to propagate the network namespace information
is ready. Make use of it.

There is a special case here between the initial network namespace and
the other namespaces:

* When ipv6 is initialized at boot time (aka in the init_net), it
registers to the notifier callback. So addrconf_notify will be called
as many time as there are network devices setup on the system and the
function will add ipv6 addresses to the network devices. But the first
device which needs to have its ipv6 address setup is the loopback,
unfortunatly this is not the case. So the loopback address is setup
manually in the ipv6 init function.

* With the network namespace, this ordering problem does not appears
because notifier is already setup and active, so as soon as we
register the loopback the ipv6 address is setup and it will be the
first device.

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