ACPI / hotplug / PCI: Execute _EJ0 under the ACPI scan lock

Since acpi_device_hotplug() assumes that ACPI handles of device
objects passed to it will not become invalid while acpi_scan_lock
is being held, make acpiphp_disable_slot() acquire acpi_scan_lock,
because it generally causes _EJ0 to be executed for one of the
devices in the slot and that may cause its ACPI handle to become
invalid.

Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>