[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: Denis V. Lunev <den@openvz.org>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/net/ipv6/addrconf.c b/net/ipv6/addrconf.c
index 4fa9da0..a65935a 100644
--- a/net/ipv6/addrconf.c
+++ b/net/ipv6/addrconf.c
@@ -2456,7 +2456,7 @@
 
 	ASSERT_RTNL();
 
-	if (dev == init_net.loopback_dev && how == 1)
+	if ((dev->flags & IFF_LOOPBACK) && how == 1)
 		how = 0;
 
 	rt6_ifdown(dev);