Fix: one NIC's IP pool shortage depleted IPs of other NICs in a multi-NIC setup.

[ty-dc]

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Notice:

• unite test or E2E test
• do not forget essential code comment and log
• document for the PR
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  "release/none"
  "release/bug"
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What issue(s) does this PR fix:

Fixes #4381

Special notes for your reviewer:

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 81.00%. Comparing base (90ac1e2) to head (b273c18).
Report is 2 commits behind head on release-v0.9.

Additional details and impacted files

@@               Coverage Diff                @@
##           release-v0.9    #4383      +/-   ##
================================================
- Coverage         81.54%   81.00%   -0.55%     
================================================
  Files                50       50              
  Lines              5446     5453       +7     
================================================
- Hits               4441     4417      -24     
- Misses              837      874      +37     
+ Partials            168      162       -6     

Flag	Coverage Δ
unittests	81.00% <100.00%> (-0.55%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
pkg/ippoolmanager/ippool_manager.go	82.96% <100.00%> (-2.21%)	⬇️

... and 1 file with indirect coverage changes