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.