PCI / ACPI fix for v4.1-rc6 This fixes a bug uncovered by a recent driver core change that modified the implementation of the ACPI_COMPANION_SET() macro to strictly rely on its second argument to be either NULL or a valid pointer to struct acpi_device. As it turns out, pcibios_root_bridge_prepare() on x86 and ia64 works with the assumption that the only code path calling pci_create_root_bus() is pci_acpi_scan_root() and therefore the sysdata argument passed to it will always match the expectations of pcibios_root_bridge_prepare(). That need not be the case, however, and in particular it is not the case for the Xen pcifront driver that passes a pointer to its own private data strcture as sysdata to pci_scan_bus_parented() which then passes it to pci_create_root_bus() and it ends up being used incorrectly by pcibios_root_bridge_prepare(). /