ACPICA: Dispatcher: Fix interpreter locking around acpi_ev_initialize_region()

In the code path of acpi_ev_initialize_region(), there is namespace
modification code unlocked. This patch tunes the code to make sure
such modification are always locked.

Fixes: 74f51b80

 (ACPICA: Namespace: Fix dynamic table loading issues)
Tested-by: Imre Deak <imre.deak@intel.com>
Signed-off-by: Lv Zheng <lv.zheng@intel.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>