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

net/ipv6/addrconf.c

@@ -3064,13 +3064,14 @@ static struct inet6_ifaddr *if6_get_first(struct seq_file *seq, loff_t pos)
 		struct hlist_node *n;
 		hlist_for_each_entry_rcu_bh(ifa, n, &inet6_addr_lst[state->bucket],
 					 addr_lst) {
+			if (!net_eq(dev_net(ifa->idev->dev), net))
+				continue;
 			/* sync with offset */
 			if (p < state->offset) {
 				p++;
 				continue;
 			}
 			state->offset++;
-			if (net_eq(dev_net(ifa->idev->dev), net))
 			return ifa;
 		}
 
@@ -3089,8 +3090,9 @@ static struct inet6_ifaddr *if6_get_next(struct seq_file *seq,
 	struct hlist_node *n = &ifa->addr_lst;
 
 	hlist_for_each_entry_continue_rcu_bh(ifa, n, addr_lst) {
+		if (!net_eq(dev_net(ifa->idev->dev), net))
+			continue;
 		state->offset++;
-		if (net_eq(dev_net(ifa->idev->dev), net))
 		return ifa;
 	}
 
@@ -3098,8 +3100,9 @@ static struct inet6_ifaddr *if6_get_next(struct seq_file *seq,
 		state->offset = 0;
 		hlist_for_each_entry_rcu_bh(ifa, n,
 				     &inet6_addr_lst[state->bucket], addr_lst) {
+			if (!net_eq(dev_net(ifa->idev->dev), net))
+				continue;
 			state->offset++;
-			if (net_eq(dev_net(ifa->idev->dev), net))
 			return ifa;
 		}
 	}