[BUG][AlluxioRuntime]Metadata not update when add/remove mountPoint in dataset

[xliuqq]

What is your environment(Kubernetes version, Fluid version, etc.) k8s 1.21.5 fluid v0.9.0 branch

Describe the bug For AlluxioRuntime, metadata(UFS TOTAL SIZE etc.) will not update when add/remove web mountPoint in dataset.

What you expect to happen: Dataset metadata should be updated.

How to reproduce it

• create dataset and alluxio runtime with yaml below.

apiVersion: data.fluid.io/v1alpha1
kind: Dataset
metadata:
  name: phy
spec:
  mounts:
    - mountPoint: https://mirrors.bit.edu.cn/apache/spark
      name: spark
---
apiVersion: data.fluid.io/v1alpha1
kind: AlluxioRuntime
metadata:
  name: phy
spec:
  replicas: 1
  tieredstore:
    levels:
      - mediumtype: MEM
        path: /dev/shm
        quota: 1Gi
        high: "0.95"
        low: "0.7"

• wait for dataset bound, get dataset show its UFS Total Size etc. metadata.
• patch dataset with adding another mountPoint https://mirrors.bit.edu.cn/apache/flink/
• wait for dataset status showing mounting two mountPoints
• get dataset show UFS Total Size is not changed.

Additional Information the metadata updating code return error if dataset has been modified, resulting in succeed mount with wrong metadata.