fix(fs): handle default skip dirs properly

[knqyf263]

Description

There are two problems. First, d.Info() could return an error on system files, like /proc. It is currently called before skipping dirs. File walking exits before processing skipping dirs. Second, user-specified skipping dirs are pre-processed, but the default skipping dirs are also processed by mistake. This PR fixes these two problems.

Related issues

• Close Trivy 0.51.0 rootfs scan throwing FATAL during vuln scan on root directory #6627

Checklist

• I've read the guidelines for contributing to this repository.
• I've followed the conventions in the PR title.
• I've added tests that prove my fix is effective or that my feature works.
• I've updated the documentation with the relevant information (if needed).
• I've added usage information (if the PR introduces new options)
• I've included a "before" and "after" example to the description (if the PR is a user interface change).