ipmi: Clear smi_info->thread to prevent use-after-free during module unload

During code inspection, I found an use-after-free possibility during unloading
ipmi_si in the polling mode.

If start_new_msg() is called after kthread_stop(), the function will try to
wake up non-existing kthread using the dangling pointer.

Possible scenario is when a new internal message is generated after
ipmi_unregister_smi()[*1] and remains after stop_timer_and_thread()
in clenaup_one_si() [*2].
Use-after-free could occur as follows depending on BMC replies.

  cleanup_one_si
    => ipmi_unregister_smi
       [*1]
    => stop_timer_and_thread
       => kthread_stop(smi_info->thread)
       [*2]
    => poll
       => smi_event_handler
          => start_new_msg
             => if (smi_info->thread)
                    wake_up_process(smi_info->thread) <== use-after-free!!

Although currently it seems no such message is generated in the polling mode,
some changes might introduce that in thefuture. For example in the interrupt
mode, disable_si_irq() does that at [*2].

So let's prevent such a critical issue possibility now.
Signed-off-by: Yamazaki Masamitsu <m-yamazaki@ah.jp.nec.com>
Signed-off-by: Corey Minyard <cminyard@mvista.com>

drivers/char/ipmi/ipmi_si_intf.c

@@ -1938,8 +1938,10 @@ static void check_for_broken_irqs(struct smi_info *smi_info)
 
 static inline void stop_timer_and_thread(struct smi_info *smi_info)
 {
-	if (smi_info->thread != NULL)
+	if (smi_info->thread != NULL) {
 		kthread_stop(smi_info->thread);
+		smi_info->thread = NULL;
+	}
 
 	smi_info->timer_can_start = false;
 	if (smi_info->timer_running)