pidfd: clone: allow CLONE_THREAD | CLONE_PIDFD together

pidfd: clone: allow CLONE_THREAD | CLONE_PIDFD together

copy_process() just needs to pass PIDFD_THREAD to __pidfd_prepare()
if clone_flags & CLONE_THREAD.

We can also add another CLONE_ flag (or perhaps reuse CLONE_DETACHED)
to enforce PIDFD_THREAD without CLONE_THREAD.

Originally-from: Tycho Andersen <tycho@tycho.pizza>
Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Link: https://lore.kernel.org/r/20240205145532.GA28823@redhat.com
Reviewed-by: Tycho Andersen <tandersen@netflix.com>
Signed-off-by: Christian Brauner <brauner@kernel.org>