selftests: add simple fsid mapping selftests

- Verify that fsid mappings cannot be written when if mappings have been
  written already.
- Set up an id mapping and an fsid mapping, create a file and compare ids in
  child and parent user namespace.

Signed-off-by: Christian Brauner <christian.brauner@ubuntu.com>
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/* v2 */
patch not present

/* v3 */
patch added
3 files changed