scripts/gdb/tasks: fix lx-ps command error

Since commit 8e1f385 ("kill task_struct->thread_group") remove
the thread_group, we will encounter below issue.

(gdb) lx-ps
      TASK          PID    COMM
0xffff800086503340   0   swapper/0
Python Exception <class 'gdb.error'>: There is no member named thread_group.
Error occurred in Python: There is no member named thread_group.

We use signal->thread_head to iterate all threads instead.

[Kuan-Ying.Lee@mediatek.com: v2]
  Link: https://lkml.kernel.org/r/20231129065142.13375-2-Kuan-Ying.Lee@mediatek.com
Link: https://lkml.kernel.org/r/20231127070404.4192-2-Kuan-Ying.Lee@mediatek.com
Fixes: 8e1f385 ("kill task_struct->thread_group")
Signed-off-by: Kuan-Ying Lee <Kuan-Ying.Lee@mediatek.com>
Acked-by: Oleg Nesterov <oleg@redhat.com>
Tested-by: Florian Fainelli <florian.fainelli@broadcom.com>
Cc: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Cc: Chinwen Chang <chinwen.chang@mediatek.com>
Cc: Kuan-Ying Lee <Kuan-Ying.Lee@mediatek.com>
Cc: Matthias Brugger <matthias.bgg@gmail.com>
Cc: Qun-Wei Lin <qun-wei.lin@mediatek.com>
Cc: Andrey Konovalov <andreyknvl@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

Author: Kuan-Ying Lee

scripts/gdb/linux/tasks.py

@@ -13,7 +13,7 @@
 
 import gdb
 
-from linux import utils
+from linux import utils, lists
 
 
 task_type = utils.CachedType("struct task_struct")
@@ -22,19 +22,15 @@
 def task_lists():
     task_ptr_type = task_type.get_type().pointer()
     init_task = gdb.parse_and_eval("init_task").address
-    t = g = init_task
+    t = init_task
 
     while True:
-        while True:
-            yield t
+        thread_head = t['signal']['thread_head']
+        for thread in lists.list_for_each_entry(thread_head, task_ptr_type, 'thread_node'):
+            yield thread
 
-            t = utils.container_of(t['thread_group']['next'],
-                                   task_ptr_type, "thread_group")
-            if t == g:
-                break
-
-        t = g = utils.container_of(g['tasks']['next'],
-                                   task_ptr_type, "tasks")
+        t = utils.container_of(t['tasks']['next'],
+                               task_ptr_type, "tasks")
         if t == init_task:
             return