ipv6: Silence privacy extensions initialization

When a network namespace is created (via CLONE_NEWNET), the loopback
interface is automatically added to the new namespace, triggering a
printk in ipv6_add_dev() if CONFIG_IPV6_PRIVACY is set.

This is problematic for applications which use CLONE_NEWNET as
part of a sandbox, like Chromium's suid sandbox or recent versions of
vsftpd. On a busy machine, it can lead to thousands of useless
"lo: Disabled Privacy Extensions" messages appearing in dmesg.

It's easy enough to check the status of privacy extensions via the
use_tempaddr sysctl, so just removing the printk seems like the most
sensible solution.

Signed-off-by: Romain Francoise <romain@orebokech.com>
Signed-off-by: David S. Miller <davem@davemloft.net>