e1000e: Avoid kernel crash during shutdown

While doing shutdown on the PCI device, the corresponding callback
function e1000e_shutdown() is trying to clear those correctable
errors on the upstream P2P bridge. Unfortunately, we don't have
the upstream P2P bridge under some cases (e.g. PCI-passthrou for
KVM on Power). That leads to kernel crash eventually.

The patch adds one more check on that to avoid kernel crash.
Signed-off-by: Li Zhang <zhlcindy@gmail.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

@@ -5999,11 +5999,18 @@ static int __e1000_shutdown(struct pci_dev *pdev, bool runtime)
 	 * correctable error when the MAC transitions from D0 to D3.  To
 	 * prevent this we need to mask off the correctable errors on the
 	 * downstream port of the pci-e switch.
+	 *
+	 * We don't have the associated upstream bridge while assigning
+	 * the PCI device into guest. For example, the KVM on power is
+	 * one of the cases.
 	 */
 	if (adapter->flags & FLAG_IS_QUAD_PORT) {
 		struct pci_dev *us_dev = pdev->bus->self;
 		u16 devctl;
 
+		if (!us_dev)
+			return 0;
+
 		pcie_capability_read_word(us_dev, PCI_EXP_DEVCTL, &devctl);
 		pcie_capability_write_word(us_dev, PCI_EXP_DEVCTL,
 					   (devctl & ~PCI_EXP_DEVCTL_CERE));