Merge pull request kubernetes#53896 from deads2k/admission-03-decode

Automatic merge from submit-queue (batch tested with PRs 47717, 53896). If you want to cherry-pick this change to another branch, please follow the instructions <a href="https://github.com/kubernetes/community/blob/master/contributors/devel/cherry-picks.md">here</a>.

decode admission responses into a fresh object

Something about the way the admission request object is built causes decoding into back into it to fail with 

```
W1013 14:10:42.457423    2960 admission.go:185] rejected by webhook namespacereservations.admission.online.openshift.io/apis/admission.online.openshift.io/v1alpha1/namespacereservations &{%!t(string=namespacereservations.admission.online.openshift.io/apis/admission.online.openshift.io/v1alpha1/namespacereservations) %!t(*errors.errorString=&{reflect.Value.Addr of unaddressable value})}: failed calling admission webhook "namespacereservations.admission.online.openshift.io/apis/admission.online.openshift.io/v1alpha1/namespacereservations": reflect.Value.Addr of unaddressable value
```

This simply creates a fresh object to decode into, which works fine for our usage and makes it possible to actually have the webhook call out to something.

Author: Kubernetes Submit Queue

plugin/pkg/admission/webhook/admission.go

@@ -242,20 +242,21 @@ func (a *GenericAdmissionWebhook) callHook(ctx context.Context, h *v1alpha1.Exte
 	if err != nil {
 		return &ErrCallingWebhook{WebhookName: h.Name, Reason: err}
 	}
-	if err := client.Post().Context(ctx).Body(&request).Do().Into(&request); err != nil {
+	response := &admissionv1alpha1.AdmissionReview{}
+	if err := client.Post().Context(ctx).Body(&request).Do().Into(response); err != nil {
 		return &ErrCallingWebhook{WebhookName: h.Name, Reason: err}
 	}
 
-	if request.Status.Allowed {
+	if response.Status.Allowed {
 		return nil
 	}
 
-	if request.Status.Result == nil {
+	if response.Status.Result == nil {
 		return fmt.Errorf("admission webhook %q denied the request without explanation", h.Name)
 	}
 
 	return &apierrors.StatusError{
-		ErrStatus: *request.Status.Result,
+		ErrStatus: *response.Status.Result,
 	}
 }