enhance: [2.5] Optimize GetLocalDiskSize and segment loader mutex

[bigsheeper]

1. Make the segment loader lock protect only the resource.
2. Optimize GetDiskUsage to avoid excessive overhead.

issue: #37630

pr: #38599

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[mergify]

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[codecov]

Codecov Report

Attention: Patch coverage is 89.83051% with 6 lines in your changes missing coverage. Please review.

Project coverage is 81.05%. Comparing base (6fa096e) to head (e64e083).

Files with missing lines	Patch %	Lines
...nternal/querynodev2/segments/disk_usage_fetcher.go	84.84%	4 Missing and 1 partial ⚠️
internal/querynodev2/segments/segment_loader.go	93.33%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##              2.5   #38907      +/-   ##
==========================================
- Coverage   81.05%   81.05%   -0.01%     
==========================================
  Files        1383     1384       +1     
  Lines      195503   195533      +30     
==========================================
+ Hits       158472   158480       +8     
- Misses      31457    31482      +25     
+ Partials     5574     5571       -3     

Components	Coverage Δ
Client	78.26% <ø> (ø)
Core	69.45% <ø> (-0.02%)	⬇️
Go	83.03% <89.83%> (-0.01%)	⬇️

Files with missing lines	Coverage Δ
internal/querynodev2/segments/segment.go	64.15% <ø> (-0.15%)	⬇️
internal/querynodev2/server.go	77.86% <100.00%> (ø)
internal/util/segcore/cgo_util.go	88.46% <100.00%> (ø)
pkg/util/paramtable/component_param.go	98.39% <100.00%> (+<0.01%)	⬆️
internal/querynodev2/segments/segment_loader.go	72.87% <93.33%> (+0.08%)	⬆️
...nternal/querynodev2/segments/disk_usage_fetcher.go	84.84% <84.84%> (ø)

... and 34 files with indirect coverage changes