Modify according to comments

1. rename zoneSeparator to gkeZoneSeparator
2. add example of regional PV's node affinity. modify test case description.

Signed-off-by: Xun Jiang <jxun@vmware.com>

pkg/backup/item_backupper.go

@@ -385,7 +385,7 @@ const (
 	awsEbsCsiZoneKey = "topology.ebs.csi.aws.com/zone"
 	azureCsiZoneKey  = "topology.disk.csi.azure.com/zone"
 	gkeCsiZoneKey    = "topology.gke.io/zone"
-	zoneSeparator    = "__"
+	gkeZoneSeparator = "__"
 )
 
 // takePVSnapshot triggers a snapshot for the volume/disk underlying a PersistentVolume if the provided
@@ -560,7 +560,7 @@ func zoneFromPVNodeAffinity(res *corev1api.PersistentVolume, topologyKeys ...str
 	}
 
 	if providerGke {
-		return gkeCsiZoneKey, strings.Join(zones, zoneSeparator)
+		return gkeCsiZoneKey, strings.Join(zones, gkeZoneSeparator)
 	}
 
 	return "", ""

pkg/backup/item_backupper_test.go

@@ -132,7 +132,22 @@ func Test_zoneFromPVNodeAffinity(t *testing.T) {
 			wantValue: "us-central",
 		},
 		{
-			name: "Volume with multiple valid keys, and provider is gke, returns the all match", // it should never happen
+			/* an valid example of node affinity in a GKE's regional PV
+			nodeAffinity:
+			  required:
+			    nodeSelectorTerms:
+			    - matchExpressions:
+			      - key: topology.gke.io/zone
+			        operator: In
+			        values:
+			        - us-central1-a
+			    - matchExpressions:
+			      - key: topology.gke.io/zone
+			        operator: In
+			        values:
+			        - us-central1-c
+			*/
+			name: "Volume with multiple valid keys, and provider is gke, returns all valid entries's first zone value",
 			pv: builder.ForPersistentVolume("multi-matching-pv").NodeAffinityRequired(
 				builder.ForNodeSelector(
 					*builder.NewNodeSelectorTermBuilder().WithMatchExpression("topology.gke.io/zone",