EXT4: Fix the construction of the path

[dkovba]

Fixes the construction of the path and removed unnecessary code. Fixes the added failing test:

✘ Test fileTreeNodePathWithAbsoluteRoot() recorded an issue at TestEXT4Reader+IO.swift:600:9: Expectation failed: (dirPtr.pointee.path → /) == (FilePath("/dir") → /dir)
✘ Test fileTreeNodePathWithAbsoluteRoot() recorded an issue at TestEXT4Reader+IO.swift:601:9: Expectation failed: (filePtr.pointee.path → /) == (FilePath("/dir/file") → /dir/file)
✘ Test fileTreeNodePathWithAbsoluteRoot() failed after 0.001 seconds with 2 issues.