ipv6: addrconf: fix /proc/net/if_inet6

Commit 1d578303 (ipv6/addrconf: speedup /proc/net/if_inet6 filling)
added bugs hiding some devices from if_inet6 and breaking applications.

"ip -6 addr" could still display all IPv6 addresses, while "ifconfig -a"
couldnt.

One way to reproduce the bug is by starting in a shell :

unshare -n /bin/bash
ifconfig lo up

And in original net namespace, lo device disappeared from if_inet6

Reported-by: Jan Hinnerk Stosch <janhinnerk.stosch@gmail.com>
Tested-by: Jan Hinnerk Stosch <janhinnerk.stosch@gmail.com>
Signed-off-by: Eric Dumazet <edumazet@google.com>
Cc: Mihai Maruseac <mihai.maruseac@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>