Update linter for all Go code

[frankyn]

While testing code changes @shubham-diwakar and I came across that the linter used for gcsfuse does not inspect existing files that are already committed and only changes in a PR.

Example:

Error: Error return value of `util.Unmount` is not checked (errcheck)
  Error: Error return value of `util.Unmount` is not checked (errcheck)
  Error: Error return value of `util.Unmount` is not checked (errcheck)
  Error: Error return value of `cmd.Wait` is not checked (errcheck)
  Error: Error return value of `cmd.Process.Kill` is not checked (errcheck)

Linter workflow needs to be updated to also lint existing code as well. This came up with in forked repo setup and filing an issue here since this is the source of truth.

[Hakai-Shin]

Hi, I’d like to take this up.
My plan is to update the GitHub Actions linter workflow so it runs on the entire codebase, and then address the existing unchecked error return values (util.Unmount, cmd.Wait, etc.) so the pipeline passes.
Please let me know if you have any preferences on how the errors should be handled

[raj-prince]

@Hakai-Shin thanks for the interest!

Ideally, we would want to run linter:

1. Only for changed files as part of pull_request event.
2. On complete codebase as part of merge/push event.

Please go ahead with the change. Thanks!

CC: @vadlakondaswetha

[Hakai-Shin]

Thank you for the clarification