fix: only remove root of SubDir when OnDelete is deleteRootSubDir

[andyzhangx]

What type of PR is this?
/kind bug

What this PR does / why we need it:
fix: only remove root of SubDir when OnDelete is deleteRootSubDir

Which issue(s) this PR fixes:

Fixes #855

Special notes for your reviewer:

Does this PR introduce a user-facing change?:

fix: only remove root of SubDir when OnDelete is deleteRootSubDir

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: andyzhangx

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• OWNERS [andyzhangx]

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[coveralls]

Pull Request Test Coverage Report for Build 13764344642

Details

• 4 of 8 (50.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-0.1%) to 77.85%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
pkg/nfs/controllerserver.go	4	8	50.0%

Totals
Change from base Build 13711291221:	-0.1%
Covered Lines:	1079
Relevant Lines:	1386

---

💛 - Coveralls