clone3: allow spawning processes into cgroups

This adds support for creating a process in a different cgroup than its
parent. Callers can limit and account processes and threads right from
the moment they are spawned:
- A service manager can directly spawn new services into dedicated
  cgroups.
- A process can be directly created in a frozen cgroup and will be
  frozen as well.
- The initial accounting jitter experienced by process supervisors and
  daemons is eliminated with this.
- Threaded applications or even thread implementations can choose to
  create a specific cgroup layout where each thread is spawned
  directly into a dedicated cgroup.

This feature is limited to the unified hierarchy. Callers need to pass
a directory file descriptor for the target cgroup. The caller can
choose to pass an O_PATH file descriptor. All usual migration
restrictions apply, i.e. there can be no processes in inner nodes. In
general, creating a process directly in a target cgroup adheres to all
migration restrictions.

One of the biggest advantages of this feature is that CLONE_INTO_GROUP does
not need to grab the write side of the cgroup cgroup_threadgroup_rwsem.
This global lock makes moving tasks/threads around super expensive. With
clone3() this lock is avoided.

Cc: Tejun Heo <[email protected]>
Cc: Ingo Molnar <[email protected]>
Cc: Oleg Nesterov <[email protected]>
Cc: Johannes Weiner <[email protected]>
Cc: Li Zefan <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Cc: [email protected]
Signed-off-by: Christian Brauner <[email protected]>
Signed-off-by: Tejun Heo <[email protected]>

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