Introduce function acpi_os_get_iomem() that may be used by its callers
to get a reference to an ACPI iomap.

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

The functions acpi_os_map_generic_address() and
acpi_os_unmap_generic_address() are only used in drivers/acpi/osl.c,
so make them static and remove the extern definitions of them from
include/linux/acpi_io.h.

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

Commit ca9b600be38c ("ACPI / PM: Make suspend_nvs_save() use
acpi_os_map_memory()") attempted to prevent the code in osl.c and nvs.c
from using different ioremap() variants by making the latter use
acpi_os_map_memory() for mapping the NVS pages.  However, that also
requires acpi_os_unmap_memory() to be used for unmapping them, which
causes synchronize_rcu() to be executed many times in a row
unnecessarily and introduces substantial delays during resume on some
systems.

Instead of using acpi_os_map_memory() for mapping the NVS pages in nvs.c
introduce acpi_os_ioremap() calling ioremap_cache() and make the code in
both osl.c and nvs.c use it.

Reported-by: Jeff Chua <jeff.chua.linux@gmail.com>
Signed-off-by: Rafael J. Wysocki <rjw@sisk.pl>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>