tuntap: orphan frags before trying to set tx timestamp

sock_tx_timestamp() will clear all zerocopy flags of skb which may lead the
frags never to be orphaned. This will break guest to guest traffic when zerocopy
is enabled. Fix this by orphaning the frags before trying to set tx time stamp.

The issue were introduced by commit eda2977291
(tun: Support software transmit time stamping).

Cc: Richard Cochran <richardcochran@gmail.com>
Cc: Sergei Shtylyov <sergei.shtylyov@cogentembedded.com>
Acked-by: Richard Cochran <richardcochran@gmail.com>
Signed-off-by: Jason Wang <jasowang@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/tun.c

@@ -749,15 +749,17 @@ static netdev_tx_t tun_net_xmit(struct sk_buff *skb, struct net_device *dev)
 			  >= dev->tx_queue_len / tun->numqueues)
 		goto drop;
 
+	if (unlikely(skb_orphan_frags(skb, GFP_ATOMIC)))
+		goto drop;
+
 	if (skb->sk) {
 		sock_tx_timestamp(skb->sk, &skb_shinfo(skb)->tx_flags);
 		sw_tx_timestamp(skb);
 	}
 
 	/* Orphan the skb - required as we might hang on to it
-	 * for indefinite time. */
-	if (unlikely(skb_orphan_frags(skb, GFP_ATOMIC)))
-		goto drop;
+	 * for indefinite time.
+	 */
 	skb_orphan(skb);
 
 	nf_reset(skb);