Call WaitGroup.Done() once only when PVB changes to fianl status the first time to avoid panic

[ywk253100]

Call WaitGroup.Done() once only when PVB changes to fianl status the first time to avoid panic

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Fixes #(issue)

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[codecov]

Codecov Report

Attention: Patch coverage is 75.00000% with 3 lines in your changes missing coverage. Please review.

Project coverage is 59.56%. Comparing base (4bd86f1) to head (f64fb36).
Report is 2 commits behind head on release-1.16.

Files with missing lines	Patch %	Lines
pkg/podvolume/backupper.go	75.00%	2 Missing and 1 partial ⚠️

Additional details and impacted files

@@              Coverage Diff              @@
##           release-1.16    #8940   +/-   ##
=============================================
  Coverage         59.56%   59.56%           
=============================================
  Files               370      370           
  Lines             40251    40262   +11     
=============================================
+ Hits              23975    23983    +8     
- Misses            14775    14777    +2     
- Partials           1501     1502    +1     

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