PCI / ACPI: hold acpi_scan_lock during root bus hotplug

During merging the PCI tree with the PM/ACPI tree, Linus noticed
that we don't use the same lock using patten about ACPI PCI root as
acpiphp.

Here apply the same locking patten, and we need to execute
acpi_bus_hot_remove_device() via acpi_os_hotplug_execute()
as it also holds acpi_scan_lock.

[rjw: Changelog]
Reported-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Yinghai Lu <yinghai@kernel.org>
No-objection-from: Bjorn Helgaas <bhelgaas@google.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>