rbd: add support for deep-flatten image feature

as deep-flatten is long supported in ceph and its
enabled by default in the librbd, providing an option
to enable it in cephcsi for the rbd images we are
creating.

Signed-off-by: Madhu Rajanna <madhupr007@gmail.com>

internal/rbd/rbd_util.go

@@ -209,6 +209,9 @@ var supportedFeatures = map[string]imageFeature{
 		needRbdNbd: true,
 		dependsOn:  []string{librbd.FeatureNameExclusiveLock},
 	},
+	librbd.FeatureNameDeepFlatten: {
+		needRbdNbd: false,
+	},
 }
 
 // GetKrbdSupportedFeatures load the module if needed and return supported