net: fix a potential rcu_read_lock() imbalance in rt6_fill_node()

Commit f2c31e32 (net: fix NULL dereferences in check_peer_redir() )
added a regression in rt6_fill_node(), leading to rcu_read_lock()
imbalance.

Thats because NLA_PUT() can make a jump to nla_put_failure label.

Fix this by using nla_put()

Many thanks to Ben Greear for his help
Reported-by: NBen Greear <greearb@candelatech.com>
Reported-by: NDave Jones <davej@redhat.com>
Signed-off-by: NEric Dumazet <eric.dumazet@gmail.com>
Tested-by: NBen Greear <greearb@candelatech.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/ipv6/route.c

@@ -2474,8 +2474,12 @@ static int rt6_fill_node(struct net *net,
 
 	rcu_read_lock();
 	n = dst_get_neighbour_noref(&rt->dst);
-	if (n)
-		NLA_PUT(skb, RTA_GATEWAY, 16, &n->primary_key);
+	if (n) {
+		if (nla_put(skb, RTA_GATEWAY, 16, &n->primary_key) < 0) {
+			rcu_read_unlock();
+			goto nla_put_failure;
+		}
+	}
 	rcu_read_unlock();
 
 	if (rt->dst.dev)