pidfs: detect refcount bugs

Now that we have pidfs_{get,register}_pid() that needs to be paired with
pidfs_put_pid() it's possible that someone pairs them with put_pid().
Thus freeing struct pid while it's still used by pidfs. Notice when that
happens. I'll also add a scheme to detect invalid uses of
pidfs_get_pid() and pidfs_put_pid() later.

Link: https://lore.kernel.org/20250506-uferbereich-guttun-7c8b1a0a431f@brauner
Signed-off-by: Christian Brauner <brauner@kernel.org>

kernel/pid.c

@@ -100,6 +100,7 @@ void put_pid(struct pid *pid)
 
 	ns = pid->numbers[pid->level].ns;
 	if (refcount_dec_and_test(&pid->count)) {
+		WARN_ON_ONCE(pid->stashed);
 		kmem_cache_free(ns->pid_cachep, pid);
 		put_pid_ns(ns);
 	}