pkg/fileutils: add verifications and unit test for SymboliLlink

[littleroad]

Ⅰ. Describe what this PR did

Ⅱ. Does this pull request fix one issue?

Ⅲ. Why don't you add test cases (unit test/integration test)? (你真的觉得不需要加测试吗？)

Ⅳ. Describe how to verify it

Ⅴ. Special notes for reviews

[pouchrobot]

We found this is your first time to contribute to Dragonfly, @littleroad
👏 We really appreciate it.
Just remind that you have read the contribution guide: https://github.com/dragonflyoss/Dragonfly/blob/master/CONTRIBUTING.md
If you didn't, you should do that first. If done, welcome again and please enjoy hacking! 🍻

[codecov-io]

Codecov Report

Merging #1223 into master will decrease coverage by 0.03%.
The diff coverage is 0%.

@@            Coverage Diff             @@
##           master    #1223      +/-   ##
==========================================
- Coverage    47.9%   47.87%   -0.04%     
==========================================
  Files         118      118              
  Lines        7436     7443       +7     
==========================================
+ Hits         3562     3563       +1     
- Misses       3588     3591       +3     
- Partials      286      289       +3

Impacted Files	Coverage Δ
pkg/fileutils/fileutils.go	74.79% <0%> (-2.79%)	⬇️
supernode/daemon/mgr/scheduler/manager.go	21.91% <0%> (-0.69%)	⬇️

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[allencloud]

LGTM