igb: Fix possible panic caused by Rx traffic arrival while interface is down

This patch reorders disabling napi and irqs during igb_down.
This is done to avoid possible panic's found in other Intel drivers
when Rx traffic arrives while interface is going down.

Signed-off-by: Carolyn Wyborny <carolyn.wyborny@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_main.c

@@ -1667,10 +1667,13 @@ void igb_down(struct igb_adapter *adapter)
 	wrfl();
 	msleep(10);
 
-	for (i = 0; i < adapter->num_q_vectors; i++)
+	igb_irq_disable(adapter);
+
+	for (i = 0; i < adapter->num_q_vectors; i++) {
+		napi_synchronize(&(adapter->q_vector[i]->napi));
 		napi_disable(&(adapter->q_vector[i]->napi));
+	}
 
-	igb_irq_disable(adapter);
 
 	del_timer_sync(&adapter->watchdog_timer);
 	del_timer_sync(&adapter->phy_info_timer);