udp: unbreak build lacking CONFIG_XFRM

We must use pre-processor conditional block or suitable accessors to
manipulate skb->sp elsewhere builds lacking the CONFIG_XFRM will break.

Fixes: dce4551c ("udp: preserve head state for IP_CMSG_PASSSEC")
Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv4/udp.c

@@ -1786,7 +1786,7 @@ static int __udp_queue_rcv_skb(struct sock *sk, struct sk_buff *skb)
 	 * the IP options and the cmsg flags, elsewhere can we clear all
 	 * pending head states while they are hot in the cache
 	 */
-	if (likely(IPCB(skb)->opt.optlen == 0 && !skb->sp))
+	if (likely(IPCB(skb)->opt.optlen == 0 && !skb_sec_path(skb)))
 		skb_release_head_state(skb);
 
 	rc = __udp_enqueue_schedule_skb(sk, skb);