enhance: 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

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[codecov]

Codecov Report

Attention: Patch coverage is 89.65517% with 6 lines in your changes missing coverage. Please review.

Project coverage is 82.95%. Comparing base (e6bf141) to head (7fc73e4).
Report is 2 commits behind head on master.

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             @@
##           master   #38599      +/-   ##
==========================================
+ Coverage   81.14%   82.95%   +1.81%     
==========================================
  Files        1387     1092     -295     
  Lines      196269   169763   -26506     
==========================================
- Hits       159257   140830   -18427     
+ Misses      31414    23346    -8068     
+ Partials     5598     5587      -11     

Components	Coverage Δ
Client	79.12% <ø> (ø)
Core	∅ <ø> (∅)
Go	83.10% <89.65%> (+<0.01%)	⬆️

Files with missing lines	Coverage Δ
internal/querynodev2/segments/segment.go	65.02% <ø> (-0.14%)	⬇️
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	72.96% <93.33%> (+0.08%)	⬆️
...nternal/querynodev2/segments/disk_usage_fetcher.go	84.84% <84.84%> (ø)

... and 323 files with indirect coverage changes

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