ftrace: Fix debug preempt config name in stack_tracer_{en,dis}able

stack_tracer_disable()/stack_tracer_enable() had been using the wrong
name for the config symbol to enable their preempt-debugging checks --
fix with a word swap.

Link: http://lkml.kernel.org/r/20170831154036.4xldyakmmhuts5x7@hatter.bewilderbeest.net

Cc: stable@vger.kernel.org
Fixes: 8aaf1ee70e ("tracing: Rename trace_active to disable_stack_tracer and inline its modification")
Signed-off-by: Zev Weiss <zev@bewilderbeest.net>
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>

include/linux/ftrace.h

@@ -307,7 +307,7 @@ DECLARE_PER_CPU(int, disable_stack_tracer);
 static inline void stack_tracer_disable(void)
 {
 	/* Preemption or interupts must be disabled */
-	if (IS_ENABLED(CONFIG_PREEMPT_DEBUG))
+	if (IS_ENABLED(CONFIG_DEBUG_PREEMPT))
 		WARN_ON_ONCE(!preempt_count() || !irqs_disabled());
 	this_cpu_inc(disable_stack_tracer);
 }
@@ -320,7 +320,7 @@ static inline void stack_tracer_disable(void)
  */
 static inline void stack_tracer_enable(void)
 {
-	if (IS_ENABLED(CONFIG_PREEMPT_DEBUG))
+	if (IS_ENABLED(CONFIG_DEBUG_PREEMPT))
 		WARN_ON_ONCE(!preempt_count() || !irqs_disabled());
 	this_cpu_dec(disable_stack_tracer);
 }