r8169: enable transmit time stamping.

This patch has been tested on a machine with the Realtek
RTL8101E/RTL8102E PCI Express Fast Ethernet controller (rev 05).

Cc: Realtek linux nic maintainers <nic_swsd@realtek.com>
Cc: Francois Romieu <romieu@fr.zoreil.com>
Signed-off-by: Richard Cochran <richardcochran@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/ethernet/realtek/r8169.c

@@ -5154,6 +5154,8 @@ static netdev_tx_t rtl8169_start_xmit(struct sk_buff *skb,
 
 	netdev_sent_queue(dev, skb->len);
 
+	skb_tx_timestamp(skb);
+
 	wmb();
 
 	/* Anti gcc 2.95.3 bugware (sic) */