amd-xgbe: Add a check for an skb in the timestamp path

Spurious Tx timestamp interrupts can cause an oops in the Tx timestamp
processing function if a Tx timestamp skb is NULL. Add a check to insure
a Tx timestamp skb is present before attempting to use it.
Signed-off-by: NTom Lendacky <thomas.lendacky@amd.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

drivers/net/ethernet/amd/xgbe/xgbe-drv.c

@@ -1212,6 +1212,10 @@ static void xgbe_tx_tstamp(struct work_struct *work)
 	u64 nsec;
 	unsigned long flags;
 
+	spin_lock_irqsave(&pdata->tstamp_lock, flags);
+	if (!pdata->tx_tstamp_skb)
+		goto unlock;
+
 	if (pdata->tx_tstamp) {
 		nsec = timecounter_cyc2time(&pdata->tstamp_tc,
 					    pdata->tx_tstamp);
@@ -1223,8 +1227,9 @@ static void xgbe_tx_tstamp(struct work_struct *work)
 
 	dev_kfree_skb_any(pdata->tx_tstamp_skb);
 
-	spin_lock_irqsave(&pdata->tstamp_lock, flags);
 	pdata->tx_tstamp_skb = NULL;
+
+unlock:
 	spin_unlock_irqrestore(&pdata->tstamp_lock, flags);
 }