ipv6: Warn users if maximum number of routes is reached.

This gives users at least some clue as to what the problem
might be and how to go about fixing it.

Signed-off-by: Ben Greear <greearb@candelatech.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv6/route.c

@@ -1945,8 +1945,12 @@ struct rt6_info *addrconf_dst_alloc(struct inet6_dev *idev,
 	struct rt6_info *rt = ip6_dst_alloc(&net->ipv6.ip6_dst_ops);
 	struct neighbour *neigh;
 
-	if (rt == NULL)
+	if (rt == NULL) {
+		if (net_ratelimit())
+			pr_warning("IPv6:  Maximum number of routes reached,"
+				   " consider increasing route/max_size.\n");
 		return ERR_PTR(-ENOMEM);
+	}
 
 	dev_hold(net->loopback_dev);
 	in6_dev_hold(idev);