fix data race reported by `go test ./... -race`

[huww98]

What type of PR is this?

Uncomment only one /kind <> line, hit enter to put that in a new line, and remove leading whitespaces from that line:

/kind api-change /kind bug

/kind cleanup

/kind design /kind documentation

/kind failing-test

/kind feature /kind flake

What this PR does / why we need it:

fix data race reported by go test ./... -race

These races should only affect test code. No actual behavior change expected.

Which issue(s) this PR fixes:

Fixes #

Special notes for your reviewer:

Does this PR introduce a user-facing change?:

NONE

[k8s-ci-robot]

Hi @huww98. Thanks for your PR.

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/lifecycle stale

[jsafrane]

/ok-to-test

[jsafrane]

/lgtm /approve

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: huww98, jsafrane

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Needs approval from an approver in each of these files: - ~~[OWNERS](https://github.com/kubernetes-csi/external-resizer/blob/master/OWNERS)~~ [jsafrane] Approvers can indicate their approval by writing `/approve` in a comment Approvers can cancel approval by writing `/approve cancel` in a comment