ip: fix warning in xfrm4_mode_tunnel_input

Same problem as IPv6
Signed-off-by: NStephen Hemminger <stephen@networkplumber.org>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/ipv4/xfrm4_mode_tunnel.c

@@ -142,7 +142,8 @@ static int xfrm4_mode_tunnel_input(struct xfrm_state *x, struct sk_buff *skb)
 	for_each_input_rcu(rcv_notify_handlers, handler)
 		handler->handler(skb);
 
-	if (err = skb_unclone(skb, GFP_ATOMIC))
+	err = skb_unclone(skb, GFP_ATOMIC);
+	if (err)
 		goto out;
 
 	if (x->props.flags & XFRM_STATE_DECAP_DSCP)