net: skbuff.h: fix using plain integer as NULL warning

Fixes the following sparse warning:
./include/linux/skbuff.h:2365:58: warning: Using plain integer as NULL pointer
Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/linux/skbuff.h

@@ -2362,7 +2362,7 @@ static inline void skb_probe_transport_header(struct sk_buff *skb,
 	if (skb_transport_header_was_set(skb))
 		return;
 
-	if (skb_flow_dissect_flow_keys_basic(skb, &keys, 0, 0, 0, 0, 0))
+	if (skb_flow_dissect_flow_keys_basic(skb, &keys, NULL, 0, 0, 0, 0))
 		skb_set_transport_header(skb, keys.control.thoff);
 	else
 		skb_set_transport_header(skb, offset_hint);