KVM: x86: fix warning Using plain integer as NULL pointer

Changed passing argument as "0 to NULL" which resolves below sparse warning

arch/x86/kvm/x86.c:3096:61: warning: Using plain integer as NULL pointer
Signed-off-by: NHariprasad Kelam <hariprasad.kelam@gmail.com>
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>

arch/x86/kvm/x86.c

@@ -3095,7 +3095,7 @@ int kvm_vm_ioctl_check_extension(struct kvm *kvm, long ext)
 		break;
 	case KVM_CAP_NESTED_STATE:
 		r = kvm_x86_ops->get_nested_state ?
-			kvm_x86_ops->get_nested_state(NULL, 0, 0) : 0;
+			kvm_x86_ops->get_nested_state(NULL, NULL, 0) : 0;
 		break;
 	default:
 		break;