KVM: x86/mmu: Cleanup sanity check of SPTEs at SP free

Massage the error message for the sanity check on SPTEs when freeing a
shadow page to be more verbose, and to print out all shadow-present SPTEs,
not just the first SPTE encountered.  Printing all SPTEs can be quite
valuable for debug, e.g. highlights whether the leak is a one-off or
widepsread, or possibly the result of memory corruption (something else
in the kernel stomping on KVM's SPTEs).

Opportunistically move the MMU_WARN_ON() into the helper itself, which
will allow a future cleanup to use BUILD_BUG_ON_INVALID() as the stub for
MMU_WARN_ON().  BUILD_BUG_ON_INVALID() works as intended and results in
the compiler complaining about is_empty_shadow_page() not being declared.

Link: https://lore.kernel.org/r/20230729004722.1056172-6-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>

arch/x86/kvm/mmu/mmu.c

@@ -1693,21 +1693,19 @@ bool kvm_test_age_gfn(struct kvm *kvm, struct kvm_gfn_range *range)
 	return young;
 }
 
-#ifdef MMU_DEBUG
-static int is_empty_shadow_page(u64 *spt)
+static void kvm_mmu_check_sptes_at_free(struct kvm_mmu_page *sp)
 {
+#ifdef MMU_DEBUG
 	int i;
 
 	for (i = 0; i < SPTE_ENT_PER_PAGE; i++) {
-		if (is_shadow_present_pte(spt[i])) {
-			printk(KERN_ERR "%s: %p %llx\n", __func__,
-			       &spt[i], spt[i]);
-			return 0;
-		}
+		if (MMU_WARN_ON(is_shadow_present_pte(sp->spt[i])))
+			pr_err_ratelimited("SPTE %llx (@ %p) for gfn %llx shadow-present at free",
+					   sp->spt[i], &sp->spt[i],
+					   kvm_mmu_page_get_gfn(sp, i));
 	}
-	return 1;
-}
 #endif
+}
 
 /*
  * This value is the sum of all of the kvm instances's
@@ -1735,7 +1733,8 @@ static void kvm_unaccount_mmu_page(struct kvm *kvm, struct kvm_mmu_page *sp)
 
 static void kvm_mmu_free_shadow_page(struct kvm_mmu_page *sp)
 {
-	MMU_WARN_ON(!is_empty_shadow_page(sp->spt));
+	kvm_mmu_check_sptes_at_free(sp);
+
 	hlist_del(&sp->hash_link);
 	list_del(&sp->link);
 	free_page((unsigned long)sp->spt);