LoongArch: Fix unreliable stack for live patching

LoongArch: Fix unreliable stack for live patching

When testing the kernel live patching with "modprobe livepatch-sample",
there is a timeout over 15 seconds from "starting patching transition"
to "patching complete". The dmesg command shows "unreliable stack" for
user tasks in debug mode, here is one of the messages:

  livepatch: klp_try_switch_task: bash:1193 has an unreliable stack

The "unreliable stack" is because it can not unwind from do_syscall()
to its previous frame handle_syscall(). It should use fp to find the
original stack top due to secondary stack in do_syscall(), but fp is
not used for some other functions, then fp can not be restored by the
next frame of do_syscall(), so it is necessary to save fp if task is
not current, in order to get the stack top of do_syscall().

Here are the call chains:

  klp_enable_patch()
    klp_try_complete_transition()
      klp_try_switch_task()
        klp_check_and_switch_task()
          klp_check_stack()
            stack_trace_save_tsk_reliable()
              arch_stack_walk_reliable()

When executing "rmmod livepatch-sample", there exists a similar issue.
With this patch, it takes a short time for patching and unpatching.

Before:

  # modprobe livepatch-sample
  # dmesg -T | tail -3
  [Sat Sep  6 11:00:20 2025] livepatch: 'livepatch_sample': starting patching transition
  [Sat Sep  6 11:00:35 2025] livepatch: signaling remaining tasks
  [Sat Sep  6 11:00:36 2025] livepatch: 'livepatch_sample': patching complete

  # echo 0 > /sys/kernel/livepatch/livepatch_sample/enabled
  # rmmod livepatch_sample
  rmmod: ERROR: Module livepatch_sample is in use
  # rmmod livepatch_sample
  # dmesg -T | tail -3
  [Sat Sep  6 11:06:05 2025] livepatch: 'livepatch_sample': starting unpatching transition
  [Sat Sep  6 11:06:20 2025] livepatch: signaling remaining tasks
  [Sat Sep  6 11:06:21 2025] livepatch: 'livepatch_sample': unpatching complete

After:

  # modprobe livepatch-sample
  # dmesg -T | tail -2
  [Tue Sep 16 16:19:30 2025] livepatch: 'livepatch_sample': starting patching transition
  [Tue Sep 16 16:19:31 2025] livepatch: 'livepatch_sample': patching complete

  # echo 0 > /sys/kernel/livepatch/livepatch_sample/enabled
  # rmmod livepatch_sample
  # dmesg -T | tail -2
  [Tue Sep 16 16:19:36 2025] livepatch: 'livepatch_sample': starting unpatching transition
  [Tue Sep 16 16:19:37 2025] livepatch: 'livepatch_sample': unpatching complete

Cc: stable@vger.kernel.org # v6.9+
Fixes: 199cc14cb4f1 ("LoongArch: Add kernel livepatching support")
Reported-by: Xi Zhang <zhangxi@kylinos.cn>
Signed-off-by: Tiezhu Yang <yangtiezhu@loongson.cn>
Signed-off-by: Huacai Chen <chenhuacai@loongson.cn>