ACPI: Make acpi_os_map_memory() avoid creating unnecessary mappings

Modify acpi_os_map_memory() so that it doesn't call acpi_os_ioremap()
unconditionally every time it is executed (except when
acpi_gbl_permanent_mmap is unset), which pretty much defeats the
purpose of maintaining the list of ACPI iomaps in osl.c.

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