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

[mergify]

@bigsheeper go-sdk check failed, comment rerun go-sdk can trigger the job again.

[codecov]

Codecov Report

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

Project coverage is 81.03%. Comparing base (54036bc) to head (c432638).
Report is 8 commits behind head on 2.5.

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   81.06%   81.03%   -0.04%     
==========================================
  Files        1399     1400       +1     
  Lines      196700   196767      +67     
==========================================
- Hits       159448   159443       -5     
- Misses      31653    31703      +50     
- Partials     5599     5621      +22     

Components	Coverage Δ
Client	78.26% <ø> (ø)
Core	69.58% <ø> (+0.04%)	⬆️
Go	82.98% <84.84%> (-0.05%)	⬇️

Files with missing lines	Coverage Δ
internal/querynodev2/segments/segment.go	66.66% <ø> (-0.14%)	⬇️
internal/querynodev2/server.go	77.45% <100.00%> (ø)
internal/util/segcore/cgo_util.go	90.32% <100.00%> (ø)
pkg/util/paramtable/component_param.go	98.39% <100.00%> (+<0.01%)	⬆️
internal/querynodev2/segments/segment_loader.go	72.63% <93.33%> (-0.08%)	⬇️
...nternal/querynodev2/segments/disk_usage_fetcher.go	84.84% <84.84%> (ø)

... and 30 files with indirect coverage changes

[mergify]

@bigsheeper go-sdk check failed, comment rerun go-sdk can trigger the job again.

[xiaofan-luan]

/lgtm
/approve

[sre-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: bigsheeper, xiaofan-luan

The full list of commands accepted by this bot can be found here.

The pull request process is described here

Needs approval from an approver in each of these files:

• OWNERS [xiaofan-luan]

Approvers can indicate their approval by writing /approve in a comment
Approvers can cancel approval by writing /approve cancel in a comment