ip_tunnel: fix possible rtable leak

ip_rt_put(rt) is always called in "error" branches above, but was missed in
skb_cow_head branch. As rt is not yet bound to skb here we have to release it by
hand.
Signed-off-by: NDmitry Popov <ixaphire@qrator.net>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/ipv4/ip_tunnel.c

@@ -668,6 +668,7 @@ void ip_tunnel_xmit(struct sk_buff *skb, struct net_device *dev,
 		dev->needed_headroom = max_headroom;
 
 	if (skb_cow_head(skb, dev->needed_headroom)) {
+		ip_rt_put(rt);
 		dev->stats.tx_dropped++;
 		kfree_skb(skb);
 		return;