KVM: SVM: Replace kmap_atomic() with kmap_local_page()

The use of kmap_atomic() is being deprecated in favor of
kmap_local_page()[1].

The main difference between atomic and local mappings is that local
mappings don't disable page faults or preemption.

There're 2 reasons we can use kmap_local_page() here:
1. SEV is 64-bit only and kmap_local_page() doesn't disable migration in
this case, but here the function clflush_cache_range() uses CLFLUSHOPT
instruction to flush, and on x86 CLFLUSHOPT is not CPU-local and flushes
the page out of the entire cache hierarchy on all CPUs (APM volume 3,
chapter 3, CLFLUSHOPT). So there's no need to disable preemption to ensure
CPU-local.
2. clflush_cache_range() doesn't need to disable pagefault and the mapping
is still valid even if sleeps. This is also true for sched out/in when
preempted.

In addition, though kmap_local_page() is a thin wrapper around
page_address() on 64-bit, kmap_local_page() should still be used here in
preference to page_address() since page_address() isn't suitable to be used
in a generic function (like sev_clflush_pages()) where the page passed in
is not easy to determine the source of allocation. Keeping the kmap* API in
place means it can be used for things other than highmem mappings[2].

Therefore, sev_clflush_pages() is a function that should use
kmap_local_page() in place of kmap_atomic().

Convert the calls of kmap_atomic() / kunmap_atomic() to kmap_local_page() /
kunmap_local().

[1]: https://lore.kernel.org/all/[email protected]
[2]: https://lore.kernel.org/lkml/[email protected]/

Suggested-by: Dave Hansen <[email protected]>
Suggested-by: Ira Weiny <[email protected]>
Suggested-by: Fabio M. De Francesco <[email protected]>
Signed-off-by: Zhao Liu <[email protected]>
Reviewed-by: Sean Christopherson <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Sean Christopherson <[email protected]>

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