fix fnplugin storagemounts validation

[totegamma]

What type of PR is this?

/kind bug

What this PR does / why we need it:

This PR fixes the path-scope validation for bind mounts.

The current implementation validates that mount paths are under the current kustomization directory with strings.HasPrefix(filepath.Clean(mount.Src), "../") but filepath.Clean("../") returns ".." so the check doesn’t work when mounts.src is exactly "../"

like

apiVersion: com.example.kustomize/v1
kind: Test
metadata:
  name: test-transformer
  annotations:
    config.kubernetes.io/function: |
      container:
        image: test
        mounts:
          - type: bind
            src: ../
            dst: /mount

https://go.dev/play/p/7YfrHGwOKnf

Other considerations

The current implementation lets '../' pass, so running kustomize build subdir instead of kustomize build . can read files in the current working directory—that is, the parent of the kustomization file—yet even this unintended reach justifies a fix.

[k8s-ci-robot]

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[k8s-ci-robot]

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