pidfs: ensure consistent ENOENT/ESRCH reporting

In a prior patch series we tried to cleanly differentiate between:

(1) The task has already been reaped.
(2) The caller requested a pidfd for a thread-group leader but the pid
    actually references a struct pid that isn't used as a thread-group
    leader.

as this was causing issues for non-threaded workloads.

But there's cases where the current simple logic is wrong. Specifically,
if the pid was a leader pid and the check races with __unhash_process().
Stabilize this by using the pidfd waitqueue lock.

Link: https://lore.kernel.org/20250411-work-pidfs-enoent-v2-2-60b2d3bb545f@kernel.org


Reviewed-by: Oleg Nesterov <oleg@redhat.com>
Signed-off-by: Christian Brauner <brauner@kernel.org>