Merge patch series "pid_namespace: make init creation more flexible"

Pavel Tikhomirov <ptikhomirov@virtuozzo.com> says:

The first patch properly annotates accesses to ->child_reaper with
_ONCE macroses, to protect unlocked accesses from possible cpu/compiler
optimization problems.

The second patch makes sure that the init is always a first process in
the pid namespace, previously this was only checked for set_tid case.

The third patch allows to join pid namespace before pid namespace init
is created, that allows to create pid namespace by one process and then
create pid namespace init from another process after setns(). Please see
the detailed description in the patch commit message. It depends on the
second patch.

The forth and the final patch is a comprehansive test, that tests both
basic usecase of creating pid namespace and init separately, and a more
specific usecase which shows how we can improve clone3(set_tid)
usability after this change.

This change is generally useful as it makes clone3(set_tid) more
universal, and let's it work in all the cases evenly. Also it is highly
useful to CRIU to handle nested containers.

* patches from https://patch.msgid.link/20260318122157.280595-1-ptikhomirov@virtuozzo.com:
  MAINTAINERS: add a new entry for testing pidns init creation via setns
  selftests: Add tests for creating pidns init via setns
  pid_namespace: allow opening pid_for_children before init was created
  pid: check init is created first after idr alloc
  pid_namespace: avoid optimization of accesses to ->child_reaper

Link: https://patch.msgid.link/20260318122157.280595-1-ptikhomirov@virtuozzo.com
Signed-off-by: Christian Brauner <brauner@kernel.org>

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