[release-21.0] Online DDL: fix defer function, potential connection pool exhaustion (#17207)

[vitess-bot]

Description

This is a backport of #17207

[vitess-bot]

Review Checklist

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General

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[codecov]

Codecov Report

Attention: Patch coverage is 0% with 1 line in your changes missing coverage. Please review.

Project coverage is 67.53%. Comparing base (cc51ed8) to head (e8357f6).
Report is 1 commits behind head on release-21.0.

Files with missing lines	Patch %	Lines
go/vt/vttablet/onlineddl/executor.go	0.00%	1 Missing ⚠️

Additional details and impacted files

@@               Coverage Diff                @@
##           release-21.0   #17210      +/-   ##
================================================
+ Coverage         67.49%   67.53%   +0.03%     
================================================
  Files              1570     1570              
  Lines            251373   251375       +2     
================================================
+ Hits             169674   169762      +88     
+ Misses            81699    81613      -86     

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