KVM: x86/mmu: Avoid pointer arithmetic when iterating over SPTEs

Replace the pointer arithmetic used to iterate over SPTEs in
is_empty_shadow_page() with more standard interger-based iteration.

No functional change intended.

Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Link: https://lore.kernel.org/r/20230729004722.1056172-5-seanjc@google.com


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