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>

test/py/tests/test_env.py

@@ -488,7 +488,7 @@ def test_env_ext4(state_test_env):
         assert 'Loading Environment from EXT4... OK' in response
 
         response = c.run_command('ext4ls host 0:0')
-        assert '8192 uboot.env' in response
+        assert '8192   uboot.env' in response
 
         response = c.run_command('env info')
         assert 'env_valid = valid' in response