generic/641: add nested user namespace tests

Test ownership and ownership changes in a complex user namespace
hierarchy.

Cc: fstests@vger.kernel.org
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Christian Brauner <christian.brauner@ubuntu.com>
---
/* v2 */
 - Eryu Guan <guan@eryu.me>:
   - Don't create symlinks for each test. Instead, use the same binary
     and introduce new options to specify which tests to run.

/* v3 */
unchanged

/* v4 */
unchanged
4 files changed