selftests/pidfd: add pidfd setns tests

This is basically a test-suite for setns() and as of now contains:
- test that we can't pass garbage flags
- test that we can't attach to the namespaces of  task that has already exited
- test that we can incrementally setns into all namespaces of a target task
  using a pidfd
- test that we can setns atomically into all namespaces of a target task
- test that we can't cross setns into a user namespace outside of our user
  namespace hierarchy
- test that we can't setns into namespaces owned by user namespaces over which
  we are not privileged

Signed-off-by: Christian Brauner <christian.brauner@ubuntu.com>
---
/* v2 */
patch introduced

/* v3 */
- Christian Brauner <christian.brauner@ubuntu.com>:
  - Minor tweaks in variable naming and a few tiny fixes.

/* v4 */
unchanged
4 files changed