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>