ACPI / dock: Take ACPI scan lock in write_undock()

Since commit 3757b948 (ACPI / hotplug: Fix concurrency issues and
memory leaks) acpi_bus_scan() and acpi_bus_trim() must always be
called under acpi_scan_lock, but currently the following scenario
violating that requirement is possible:

 write_undock()
  handle_eject_request()
   hotplug_dock_devices()
    dock_remove_acpi_device()
     acpi_bus_trim()

Fix that by making write_undock() acquire acpi_scan_lock before
calling handle_eject_request() as appropriate (begin_undock() is
under the lock too in analogy with acpi_dock_deferred_cb()).

Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Cc: 3.9+ <stable@vger.kernel.org>
Acked-by: Toshi Kani <toshi.kani@hp.com>