xfrm: force a dst reference in __xfrm_route_forward()

Packets going through __xfrm_route_forward() have a not refcounted dst
entry, since we enabled a noref forwarding path.

xfrm_lookup() might incorrectly release this dst entry.

It's a bit late to make invasive changes in xfrm_lookup(), so lets force
a refcount in this path.
Signed-off-by: NEric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/xfrm/xfrm_policy.c

@@ -2153,6 +2153,7 @@ int __xfrm_route_forward(struct sk_buff *skb, unsigned short family)
 		return 0;
 	}
 
+	skb_dst_force(skb);
 	dst = skb_dst(skb);
 
 	res = xfrm_lookup(net, &dst, &fl, NULL, 0) == 0;