KVM: sefltests: Verify value of dirty_log_test last page isn't bogus

Add a sanity check that a completely garbage value wasn't written to
the last dirty page in the ring, e.g. that it doesn't contain the *next*
iteration's value.

Reviewed-by: Maxim Levitsky <mlevitsk@redhat.com>
Link: https://lore.kernel.org/r/20250111003004.1235645-16-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>

Author: sean-jc

tools/testing/selftests/kvm/dirty_log_test.c

@@ -514,8 +514,9 @@ static void vm_dirty_log_verify(enum vm_guest_mode mode, unsigned long **bmap)
 				 * last page's iteration), as the value to be
 				 * written may be cached in a CPU register.
 				 */
-				if (page == dirty_ring_last_page ||
-				    page == dirty_ring_prev_iteration_last_page)
+				if ((page == dirty_ring_last_page ||
+				     page == dirty_ring_prev_iteration_last_page) &&
+				    val < iteration)
 					continue;
 			} else if (!val && iteration == 1 && bmap0_dirty) {
 				/*