Use mutex to protect acpi_pci_roots list against PCI host bridge
hotplug operations.

[bhelgaas: s/struct acpi_handle *handle/acpi_handle handle/]
Signed-off-by: NTaku Izumi <izumi.taku@jp.fujitsu.com>
Signed-off-by: NBjorn Helgaas <bhelgaas@google.com>