add gocritic/ruleguard checks back again, cleanup code. (#13665)

- remove some duplicated code
- reported a bug, separately fixed in #13664
- using strings.ReplaceAll() when needed
- using filepath.ToSlash() use when needed
- remove all non-Go style comments from the codebase

Co-authored-by: Aditya Manthramurthy <donatello@users.noreply.github.com>

cmd/fs-v1-helpers.go

@@ -327,7 +327,7 @@ func fsCreateFile(ctx context.Context, filePath string, reader io.Reader, falloc
 
 	flags := os.O_CREATE | os.O_WRONLY
 	if globalFSOSync {
-		flags = flags | os.O_SYNC
+		flags |= os.O_SYNC
 	}
 	writer, err := lock.Open(filePath, flags, 0666)
 	if err != nil {