ACPI: Avoid walking the list of memory mappings in osl.c twice in a row

Make acpi_os_unmap_generic_address() use acpi_map_lookup() to find
the desired iomap and drop the reference to it directly (and
eventually remove it if necessary) instead of calling
acpi_os_unmap_memory(), which requires us to walk the list of ACPI
iomaps twice in a row (first, to get the virtual address associated
with the iomap and second, to get the iomap itself).

Signed-off-by: Rafael J. Wysocki <rjw@sisk.pl>