PCI: Don't oops on virtual buses in acpi_pci_get_bridge_handle()

acpi_pci_get_bridge_handle() returns the ACPI handle for the bridge device
(either a host bridge or a PCI-to-PCI bridge) leading to a PCI bus.  But
SR-IOV virtual functions can be on a virtual bus with no bridge leading to
it.  Return a NULL acpi_handle in this case instead of trying to
dereference the NULL pointer to the bridge.

This fixes a NULL pointer dereference oops in pci_get_hp_params() when
adding SR-IOV VF devices on virtual buses.

[bhelgaas: changelog, add comment in code]
Fixes: 6cd33649 ("PCI: Add pci_configure_device() during enumeration")
Link: https://bugzilla.kernel.org/show_bug.cgi?id=87591Reported-by: NChao Zhou <chao.zhou@intel.com>
Reported-by: NJoerg Roedel <joro@8bytes.org>
Signed-off-by: NYinghai Lu <yinghai@kernel.org>
Signed-off-by: NBjorn Helgaas <bhelgaas@google.com>

include/linux/pci-acpi.h

@@ -41,8 +41,13 @@ static inline acpi_handle acpi_pci_get_bridge_handle(struct pci_bus *pbus)
 
 	if (pci_is_root_bus(pbus))
 		dev = pbus->bridge;
-	else
+	else {
+		/* If pbus is a virtual bus, there is no bridge to it */
+		if (!pbus->self)
+			return NULL;
+
 		dev = &pbus->self->dev;
+	}
 
 	return ACPI_HANDLE(dev);
 }