builder: correctly generate nid for v6 inodes

[jiangliu]

The nid is not actually used yet, but we should still generate it with correct value.

Relevant Issue (if applicable)

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Details

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Types of changes

What types of changes does your PullRequest introduce? Put an x in all the boxes that apply:

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation Update (if none of the other choices apply)

Checklist

Go over all the following points, and put an x in all the boxes that apply.

• I have updated the documentation accordingly.
• I have added tests to cover my changes.

[anolis-bot]

@jiangliu , a new test job has been submitted. Please wait in patience. The test job url: https://tone.openanolis.cn/ws/nrh4nnio/test_result/75716

[codecov]

Codecov Report

Merging #1304 (ccdd579) into master (23c6110) will decrease coverage by 0.02%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##           master    #1304      +/-   ##
==========================================
- Coverage   45.93%   45.91%   -0.02%     
==========================================
  Files         123      123              
  Lines       37028    37031       +3     
  Branches    37028    37031       +3     
==========================================
- Hits        17009    17004       -5     
- Misses      19125    19130       +5     
- Partials      894      897       +3     

Impacted Files	Coverage Δ
rafs/src/builder/core/bootstrap.rs	72.84% <0.00%> (ø)
rafs/src/builder/core/v6.rs	32.45% <0.00%> (-0.12%)	⬇️
rafs/src/builder/stargz.rs	0.00% <0.00%> (ø)

... and 4 files with indirect coverage changes

[anolis-bot]

@jiangliu , the code has been updated, so a new test job has been submitted. Please wait in patience. The test job url: https://tone.openanolis.cn/ws/nrh4nnio/test_result/75717

[anolis-bot]

@jiangliu , The CI test is completed, please check result:

Test Case		Test Result
build rust golang image		✅ SUCCESS
compile nydusd		✅ SUCCESS
compile ctr remote		✅ SUCCESS
compile nydus snapshotter		✅ SUCCESS
run container with rafs		✅ SUCCESS
run container with zran		✅ SUCCESS
run container with rafs and compile linux		✅ SUCCESS

Congratulations, your test job passed!

[anolis-bot]

@jiangliu , The CI test is completed, please check result:

Test Case		Test Result
nydus_ci		❌ FAIL

Sorry, your test job failed. Please get the details in the link.