x86/access: Don't write page tables if desired PTE is same as current PTE

Avoid spurious PTE writes, KVM doesn't check if old==new when handling
write-protected SPTEs and triggers an MMU sync when using shadow paging
even if the SPTE is unchanged.

Signed-off-by: Sean Christopherson <seanjc@google.com>
Message-Id: <20211125012857.508243-15-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>