x86/ACPI: fix S3 wakeup vector mapping

x86/ACPI: fix S3 wakeup vector mapping

Use of __acpi_map_table() here was at least close to an abuse already
before, but it will now consistently return NULL here. Drop the layering
violation and use set_fixmap() directly. Re-use of the ACPI fixmap area
is hopefully going to remain "fine" for the time being.

Add checks to acpi_enter_sleep(): The vector now needs to be contained
within a single page, but the ACPI spec requires 64-byte alignment of
FACS anyway. Also bail if no wakeup vector was determined in the first
place, in part as preparation for a subsequent relaxation change.

Fixes: 1c4aa69ca1e1 ("xen/acpi: Rework acpi_os_map_memory() and acpi_os_unmap_memory()")
Signed-off-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Roger Pau Monné <roger.pau@citrix.com>