procfs: fix missing RCU protection when reading real_parent in do_task_stat()

When reading /proc/[pid]/stat, do_task_stat() accesses task->real_parent
without proper RCU protection, which leads to:

  cpu 0                               cpu 1
  -----                               -----
  do_task_stat
    var = task->real_parent
                                      release_task
                                        call_rcu(delayed_put_task_struct)
    task_tgid_nr_ns(var)
      rcu_read_lock   <--- Too late to protect task->real_parent!
      task_pid_ptr    <--- UAF!
      rcu_read_unlock

This patch uses task_ppid_nr_ns() instead of task_tgid_nr_ns() to add
proper RCU protection for accessing task->real_parent.

Link: https://lkml.kernel.org/r/20260128083007.3173016-1-alexjlzheng@tencent.com
Fixes: 06fffb1267 ("do_task_stat: don't take rcu_read_lock()")
Signed-off-by: Jinliang Zheng <alexjlzheng@tencent.com>
Acked-by: Oleg Nesterov <oleg@redhat.com>
Cc: David Hildenbrand <david@kernel.org>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
Cc: Mateusz Guzik <mjguzik@gmail.com>
Cc: ruippan <ruippan@tencent.com>
Cc: Usama Arif <usamaarif642@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

fs/proc/array.c

@@ -529,7 +529,7 @@ static int do_task_stat(struct seq_file *m, struct pid_namespace *ns,
 		}
 
 		sid = task_session_nr_ns(task, ns);
-		ppid = task_tgid_nr_ns(task->real_parent, ns);
+		ppid = task_ppid_nr_ns(task, ns);
 		pgid = task_pgrp_nr_ns(task, ns);
 
 		unlock_task_sighand(task, &flags);