Remove possible nil pointer dereference in mkdir

[cbartoszDell]

Description

There was a possibility for nil pointer dereference on os.Stat result. This is now fixed. Also refactored the function so there is less nesting and happy path is the default.

dell/csm#1014

Checklist:

• I have performed a self-review of my own code to ensure there are no formatting, vetting, linting, or security issues
• I have verified that new and existing unit tests pass locally with my changes
• I have not allowed coverage numbers to degenerate
• I have maintained at least 90% code coverage
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have added tests that prove my fix is effective or that my feature works
• Backward compatibility is not broken

How Has This Been Tested?

I have crated test cases for all but one path in the function, because I haven't found a way to trigger fs errors other than fs.ErrNotFound easily. The logic is pretty simple so it should be correct.

[gallacher]

Is this a fix related to a GitHub bug? Or part of feature implementation? Please associate this PR with a GitHub issue. Thanks.

[gallacher]

Is this code change fixing a defect? Please associate it with a GitHub issue. Thanks.