scripts/gdb/linux/tasks.py: fix get_thread_info

Since kernel 4.9, the thread_info has been moved into task_struct, no
longer locates at the bottom of kernel stack.

See commits c65eacbe ("sched/core: Allow putting thread_info into
task_struct") and 15f4eae7 ("x86: Move thread_info into
task_struct").

Before fix:
  (gdb) set $current = $lx_current()
  (gdb) p $lx_thread_info($current)
  $1 = {flags = 1470918301}
  (gdb) p $current.thread_info
  $2 = {flags = 2147483648}

After fix:
  (gdb) p $lx_thread_info($current)
  $1 = {flags = 2147483648}
  (gdb) p $current.thread_info
  $2 = {flags = 2147483648}

Link: http://lkml.kernel.org/r/20180118210159.17223-1-imxikangjie@gmail.com
Fixes: 15f4eae7 ("x86: Move thread_info into task_struct")
Signed-off-by: Xi Kangjie <imxikangjie@gmail.com>
Acked-by: Jan Kiszka <jan.kiszka@siemens.com>
Acked-by: Kieran Bingham <kbingham@kernel.org>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

scripts/gdb/linux/tasks.py

@@ -96,6 +96,8 @@ def get_thread_info(task):
         thread_info_addr = task.address + ia64_task_size
         thread_info = thread_info_addr.cast(thread_info_ptr_type)
     else:
+        if task.type.fields()[0].type == thread_info_type.get_type():
+            return task['thread_info']
         thread_info = task['stack'].cast(thread_info_ptr_type)
     return thread_info.dereference()