Fix mountpoint multiple match

[coco-roll]

When use make test base on this case
8 5 0:7 /docker /sys/fs/cgroup/memory rw,nosuid,nodev,noexec,relatime shared:8 - cgroup cgroup rw,memory 9 5 0:8 /docker/memory.vmstat /sys/fs/cgroup/vmstat rw,nosuid,nodev,noexec,relatime shared:8 - cgroup cgroup rw,memory
been failed.

Because it's match multiple target，cannot uniquely match

[CLAassistant]

All committers have signed the CLA.

[coco-roll]

fix #41

[codecov]

Codecov Report

❌ Patch coverage is 60.00000% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 91.01%. Comparing base (0180b04) to head (57563fa).
⚠️ Report is 27 commits behind head on master.

Files with missing lines	Patch %	Lines
internal/cgroups/mountpoint.go	60.00%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #42      +/-   ##
==========================================
- Coverage   92.07%   91.01%   -1.06%     
==========================================
  Files           8        8              
  Lines         164      167       +3     
==========================================
+ Hits          151      152       +1     
- Misses         10       11       +1     
- Partials        3        4       +1     

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[coco-roll]

Why Go / build (1.16.x) (pull_request) Failing?
I was run: make test and make lint all passed