e1000e: panic caused by Rx traffic arriving while interface going down

An "unable to handle kernel paging request" panic can occur when receiving
traffic while the interface is going down.  Wait for NAPI to be done with
current context after disabling interrupts and then disable NAPI.

See https://bugzilla.vyatta.com/show_bug.cgi?id=8837.
Reported-by: Stephen Hemminger <stephen@networkplumber.org>
Signed-off-by: Bruce Allan <bruce.w.allan@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/e1000e/netdev.c

@@ -4016,6 +4016,8 @@ void e1000e_down(struct e1000_adapter *adapter)
 
 	e1000_irq_disable(adapter);
 
+	napi_synchronize(&adapter->napi);
+
 	del_timer_sync(&adapter->watchdog_timer);
 	del_timer_sync(&adapter->phy_info_timer);
 
@@ -4372,12 +4374,13 @@ static int e1000_close(struct net_device *netdev)
 
 	pm_runtime_get_sync(&pdev->dev);
 
-	napi_disable(&adapter->napi);
-
 	if (!test_bit(__E1000_DOWN, &adapter->state)) {
 		e1000e_down(adapter);
 		e1000_free_irq(adapter);
 	}
+
+	napi_disable(&adapter->napi);
+
 	e1000_power_down_phy(adapter);
 
 	e1000e_free_tx_resources(adapter->tx_ring);