netns: fix addrconf_ifdown kernel panic

When a network namespace is destroyed the network interfaces are
all unregistered, making addrconf_ifdown called by the netdevice
notifier. 
In the other hand, the addrconf exit method does a loop on the network
devices and does addrconf_ifdown on each of them. But the ordering of 
the netns subsystem is not right because it uses the register_pernet_device
instead of register_pernet_subsys. If we handle the loopback as
any network device, we can safely use register_pernet_subsys.

But if we use register_pernet_subsys, the addrconf exit method will do
exactly what was already done with the unregistering of the network
devices. So in definitive, this code is pointless.

I removed the netns addrconf exit method and moved the code to the
addrconf cleanup function.

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