x86/access: Preserve A/D bits when writing paging structure entries

Preserve A/D bits for paging structure entries to avoid pointless writes
to PTEs between test iterations, and more importantly to avoid triggering
MMU syncs due to writing upper-level PTEs.

Signed-off-by: Sean Christopherson <seanjc@google.com>
Message-Id: <20211125012857.508243-16-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>