ipv4: Give backtrace in ip_rt_bug().

Add a stack backtrace to the ip_rt_bug path for debugging

Signed-off-by: Dave Jones <davej@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/net/ipv4/route.c b/net/ipv4/route.c
index b24d58e..52b0b95 100644
--- a/net/ipv4/route.c
+++ b/net/ipv4/route.c
@@ -1665,6 +1665,7 @@
 		&ip_hdr(skb)->saddr, &ip_hdr(skb)->daddr,
 		skb->dev ? skb->dev->name : "?");
 	kfree_skb(skb);
+	WARN_ON(1);
 	return 0;
 }