KVM: x86/mmu: Always update A/D-disabled SPTEs atomically

KVM: x86/mmu: Always update A/D-disabled SPTEs atomically

In anticipation of aging SPTEs outside of mmu_lock, force A/D-disabled
SPTEs to be updated atomically, as aging A/D-disabled SPTEs will mark them
for access-tracking outside of mmu_lock.  Coupled with restoring access-
tracked SPTEs in the fast page fault handler, the end result is that
A/D-disable SPTEs will be volatile at all times.

Reviewed-by: James Houghton <jthoughton@google.com>
Link: https://lore.kernel.org/all/Z60bhK96JnKIgqZQ@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>