[BUG][AlluxioRuntime] multiple mountPoint containing root path is not correct mounted in alluxio

[xliuqq]

What is your environment(Kubernetes version, Fluid version, etc.) k8s 1.21, fluid 0.9

Describe the bug multiple mountPoint containing root path '/' is not correct mounted in alluxio.

• create dataset and runtime using below yaml

apiVersion: data.fluid.io/v1alpha1
kind: Dataset
metadata:
  name: phy
spec:
  mounts:
    - mountPoint: https://mirrors.bit.edu.cn/apache/spark/
      name: spark
    - mountPoint: https://mirrors.bit.edu.cn/apache/flink/
      path: /
      name: flink
---
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"

execute alluxio fs mount command in master pod, output is

https://mirrors.bit.edu.cn/apache/spark  on  /spark  (web, capacity=-1B, used=-1B, read-only, not shared, properties={})
/underFSStorage                          on  /       (local, capacity=0B, used=0B, not read-only, not shared, properties={})

What you expect to happen: the root path should mount on https://mirrors.bit.edu.cn/apache/flink/ not /.

Additional Information related code ignores root path mount when have multiple mountPoints.