igb: Free any held skb that should have been timestamped on remove

To prevent a race condition where an skb has been saved to return the Tx
timestamp later and the driver is removed, add a check to determine if we
have an skb stored and, if so, free it.

Cc: Richard Cochran <richardcochran@gmail.com>
Signed-off-by: Matthew Vick <matthew.vick@intel.com>
Acked-by: Jacob Keller <Jacob.e.keller@intel.com>
Tested-by: Aaron Brown <aaron.f.brown@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>

drivers/net/ethernet/intel/igb/igb_ptp.c

@@ -856,6 +856,10 @@ void igb_ptp_stop(struct igb_adapter *adapter)
 	}
 
 	cancel_work_sync(&adapter->ptp_tx_work);
+	if (adapter->ptp_tx_skb) {
+		dev_kfree_skb_any(adapter->ptp_tx_skb);
+		adapter->ptp_tx_skb = NULL;
+	}
 
 	if (adapter->ptp_clock) {
 		ptp_clock_unregister(adapter->ptp_clock);