ipv6: use xarray iterator to implement inet6_dump_ifinfo()

Prepare inet6_dump_ifinfo() to run with RCU protection
instead of RTNL and use for_each_netdev_dump() interface.

Also properly return 0 at the end of a dump, avoiding
an extra recvmsg() system call and RTNL acquisition.

Note that RTNL-less dumps need core changes, coming later
in the series.
Signed-off-by: Eric Dumazet <edumazet@google.com>
Reviewed-by: Ido Schimmel <idosch@nvidia.com>
Reviewed-by: Donald Hunter <donald.hunter@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv6/addrconf.c

@@ -6132,50 +6132,42 @@ static int inet6_valid_dump_ifinfo(const struct nlmsghdr *nlh,
 static int inet6_dump_ifinfo(struct sk_buff *skb, struct netlink_callback *cb)
 {
 	struct net *net = sock_net(skb->sk);
-	int h, s_h;
-	int idx = 0, s_idx;
+	struct {
+		unsigned long ifindex;
+	} *ctx = (void *)cb->ctx;
 	struct net_device *dev;
 	struct inet6_dev *idev;
-	struct hlist_head *head;
+	int err;
 
 	/* only requests using strict checking can pass data to
 	 * influence the dump
 	 */
 	if (cb->strict_check) {
-		int err = inet6_valid_dump_ifinfo(cb->nlh, cb->extack);
+		err = inet6_valid_dump_ifinfo(cb->nlh, cb->extack);
 
 		if (err < 0)
 			return err;
 	}
 
-	s_h = cb->args[0];
-	s_idx = cb->args[1];
-
+	err = 0;
 	rcu_read_lock();
-	for (h = s_h; h < NETDEV_HASHENTRIES; h++, s_idx = 0) {
-		idx = 0;
-		head = &net->dev_index_head[h];
-		hlist_for_each_entry_rcu(dev, head, index_hlist) {
-			if (idx < s_idx)
-				goto cont;
+	for_each_netdev_dump(net, dev, ctx->ifindex) {
 		idev = __in6_dev_get(dev);
 		if (!idev)
-				goto cont;
-			if (inet6_fill_ifinfo(skb, idev,
+			continue;
+		err = inet6_fill_ifinfo(skb, idev,
 					NETLINK_CB(cb->skb).portid,
 					cb->nlh->nlmsg_seq,
-					      RTM_NEWLINK, NLM_F_MULTI) < 0)
-				goto out;
-cont:
-			idx++;
+					RTM_NEWLINK, NLM_F_MULTI);
+		if (err < 0) {
+			if (likely(skb->len))
+				err = skb->len;
+			break;
 		}
 	}
-out:
 	rcu_read_unlock();
-	cb->args[1] = idx;
-	cb->args[0] = h;
 
-	return skb->len;
+	return err;
 }
 
 void inet6_ifinfo_notify(int event, struct inet6_dev *idev)