tracing: Fix check of ftrace_trace_arrays list_empty() check

The check that tests if ftrace_trace_arrays is empty in
top_trace_array(), uses the .prev pointer:

  if (list_empty(ftrace_trace_arrays.prev))

instead of testing the variable itself:

  if (list_empty(&ftrace_trace_arrays))

Although it is technically correct, it is awkward and confusing.
Use the proper method.

Link: http://lkml.kernel.org/r/87oay1bas8.fsf@sejong.aot.lge.com

Reported-by: Namhyung Kim <namhyung@gmail.com>
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>

kernel/trace/trace.h

@@ -252,7 +252,7 @@ static inline struct trace_array *top_trace_array(void)
 {
 	struct trace_array *tr;
 
-	if (list_empty(ftrace_trace_arrays.prev))
+	if (list_empty(&ftrace_trace_arrays))
 		return NULL;
 
 	tr = list_entry(ftrace_trace_arrays.prev,