[IPV6]: inet6_dev on loopback should be kept until namespace stop.

In the other case it will be destroyed when last address will be removed
from lo inside a namespace. This will break IPv6 in several places. The
most obvious one is ip6_dst_ifdown.
Signed-off-by: NDenis V. Lunev <den@openvz.org>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/ipv6/addrconf.c

@@ -2456,7 +2456,7 @@ static int addrconf_ifdown(struct net_device *dev, int how)
 
 	ASSERT_RTNL();
 
-	if (dev == init_net.loopback_dev && how == 1)
+	if ((dev->flags & IFF_LOOPBACK) && how == 1)
 		how = 0;
 
 	rt6_ifdown(dev);