Online DDL: fix defer function, potential connection pool exhaustion

[shlomi-noach]

Description

Quick fix to the location of defer lockConn.Recycle() in cutOverVReplMigration(): should be written immediately after the connection has been acquired. As it stands right now, it is possible for the function to return error because the defer has been declared, leading to connection not getting recycled.

To be backported to supported versions

Related Issue(s)

Fixes #17206

Checklist

• "Backport to:" labels have been added if this change should be back-ported to release branches
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• Tests were added or are not required
• Did the new or modified tests pass consistently locally and on CI?
• Documentation was added or is not required

Deployment Notes

[vitess-bot]

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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 67.34%. Comparing base (0403d54) to head (d554a1a).
Report is 1 commits behind head on main.

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

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #17207      +/-   ##
==========================================
- Coverage   67.34%   67.34%   -0.01%     
==========================================
  Files        1570     1570              
  Lines      252734   252736       +2     
==========================================
+ Hits       170204   170205       +1     
- Misses      82530    82531       +1     

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