acpi/apei: Use appropriate pgprot_t to map GHES memory

If the ACPI APEI firmware handles hardware error first (called
"firmware first handling"), the firmware updates the GHES memory
region with hardware error record (called "generic hardware
error record"). Essentially the firmware writes hardware error
records in the GHES memory region, triggers an NMI/interrupt,
then the GHES driver goes off and grabs the error record from
the GHES region.

The kernel currently maps the GHES memory region as cacheable
(PAGE_KERNEL) for all architectures. However, on some arm64
platforms, there is a mismatch between how the kernel maps the
GHES region (PAGE_KERNEL) and how the firmware maps it
(EFI_MEMORY_UC, ie. uncacheable), leading to the possibility of
the kernel GHES driver reading stale data from the cache when it
receives the interrupt.

With stale data being read, the kernel is unaware there is new
hardware error to be handled when there actually is; this may
lead to further damage in various scenarios, such as error
propagation caused data corruption. If uncorrected error (such
as double bit ECC error) happened in memory operation and if the
kernel is unaware of such an event happening, errorneous data may
be propagated to the disk.

Instead GHES memory region should be mapped with page protection
type according to what is returned from arch_apei_get_mem_attribute().

Signed-off-by: Jonathan (Zhixiong) Zhang <zjzhang@codeaurora.org>
Signed-off-by: Matt Fleming <matt.fleming@intel.com>
[ Small stylistic tweaks. ]
Reviewed-by: Matt Fleming <matt@codeblueprint.co.uk>
Acked-by: Borislav Petkov <bp@suse.de>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: http://lkml.kernel.org/r/1441372302-23242-3-git-send-email-matt@codeblueprint.co.uk
Signed-off-by: Ingo Molnar <mingo@kernel.org>

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