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/20220813220034.806698-1-ira.weiny@intel.com
[2]: https://lore.kernel.org/lkml/5d667258-b58b-3d28-3609-e7914c99b31b@intel.com/

Suggested-by: Dave Hansen <dave.hansen@intel.com>
Suggested-by: Ira Weiny <ira.weiny@intel.com>
Suggested-by: Fabio M. De Francesco <fmdefrancesco@gmail.com>
Signed-off-by: Zhao Liu <zhao1.liu@intel.com>
Reviewed-by: Sean Christopherson <seanjc@google.com>
Link: https://lore.kernel.org/r/20220928092748.463631-1-zhao1.liu@linux.intel.com
Signed-off-by: Sean Christopherson <seanjc@google.com>