r8169: remove unneeded memory barrier in rtl_tx

tp->dirty_tx isn't changed outside rtl_tx(). Therefore I see no need
to guarantee a specific order of reading tp->dirty_tx and tp->cur_tx.
Having said that we can remove the memory barrier.
In addition use READ_ONCE() when reading tp->cur_tx because it can
change in parallel to rtl_tx().
Signed-off-by: Heiner Kallweit <hkallweit1@gmail.com>
Link: https://lore.kernel.org/r/2264563a-fa9e-11b0-2c42-31bc6b8e2790@gmail.comSigned-off-by: Jakub Kicinski <kuba@kernel.org>

drivers/net/ethernet/realtek/r8169_main.c

@@ -4364,9 +4364,8 @@ static void rtl_tx(struct net_device *dev, struct rtl8169_private *tp,
 	unsigned int dirty_tx, tx_left, bytes_compl = 0, pkts_compl = 0;
 
 	dirty_tx = tp->dirty_tx;
-	smp_rmb();
 
-	for (tx_left = tp->cur_tx - dirty_tx; tx_left > 0; tx_left--) {
+	for (tx_left = READ_ONCE(tp->cur_tx) - dirty_tx; tx_left; tx_left--) {
 		unsigned int entry = dirty_tx % NUM_TX_DESC;
 		struct sk_buff *skb = tp->tx_skb[entry].skb;
 		u32 status;