fs: ext4: use fs_ls_generic

fs: ext4: use fs_ls_generic

Now that opendir, readir, closedir are implemented for ext4 we can use
fs_ls_generic() for implementing the ls command.

Adjust the unit tests:

* fs_ls_generic() produces more spaces between file size and name.
* The ext4 specific message "** Can not find directory. **\n" is not
  written anymore.

Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>
Reviewed-by: Simon Glass <sjg@chromium.org>