KVM: x86: Sanity check that kvm_has_noapic_vcpu is zero at module_exit()

WARN if kvm.ko is unloaded with an elevated kvm_has_noapic_vcpu to guard
against incorrect management of the key, e.g. to detect if KVM fails to
decrement the key in error paths.  Because kvm_has_noapic_vcpu is purely
an optimization, in all likelihood KVM could completely botch handling of
kvm_has_noapic_vcpu and no one would notice (which is a good argument for
deleting the key entirely, but that's a problem for another day).

Note, ideally the sanity check would be performance when kvm_usage_count
goes to zero, but adding an arch callback just for this sanity check isn't
at all worth doing.

Link: https://lore.kernel.org/r/20240209222047.394389-3-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>

arch/x86/kvm/x86.c

@@ -13871,9 +13871,6 @@ module_init(kvm_x86_init);
 
 static void __exit kvm_x86_exit(void)
 {
-	/*
-	 * If module_init() is implemented, module_exit() must also be
-	 * implemented to allow module unload.
-	 */
+	WARN_ON_ONCE(static_branch_unlikely(&kvm_has_noapic_vcpu));
 }
 module_exit(kvm_x86_exit);