x86/stacktrace: return 0 instead of -1 for stack ops

If we return -1 in the ops->stack for the stacktrace saving, we end up
breaking out of the loop if the stack we are tracing is in the exception
stack. This causes traces like:

          <idle>-0     [002] 34263.745825: raise_softirq_irqoff <-__blk_complete_request
          <idle>-0     [002] 34263.745826:
 <= 0
 <= 0
 <= 0
 <= 0
 <= 0
 <= 0
 <= 0

By returning "0" instead, the irq stack is saved as well, and we see:

          <idle>-0     [003]   883.280992: raise_softirq_irqoff <-__hrtimer_star
t_range_ns
          <idle>-0     [003]   883.280992:
 <= hrtimer_start_range_ns
 <= tick_nohz_restart_sched_tick
 <= cpu_idle
 <= start_secondary
 <=
 <= 0
 <= 0

[ Impact: record stacks from interrupts ]
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>

arch/x86/kernel/stacktrace.c

@@ -20,7 +20,7 @@ save_stack_warning_symbol(void *data, char *msg, unsigned long symbol)
 
 static int save_stack_stack(void *data, char *name)
 {
-	return -1;
+	return 0;
 }
 
 static void save_stack_address(void *data, unsigned long addr, int reliable)