There is no reason (knwon to me) why any of the existing users of
acpi_os_unmap_iomem() would need to wait for the unused memory
mappings left by it to actually go away, so use the deferred
unmapping of ACPI memory introduced previously in that function.

While at it, fold __acpi_os_unmap_iomem() back into
acpi_os_unmap_iomem(), which has become a simple wrapper around it,
and make acpi_os_unmap_memory() call the latter.
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>