tracing: Fix the file mode of stack tracer

It looks weird that the stack_trace_filter file can be written by root
but shows that it does not have write permission by ll command.

Link: http://lkml.kernel.org/r/1518054113-28096-1-git-send-email-liuzhengyuan@kylinos.cnSigned-off-by: Zhengyuan Liu <liuzhengyuan@kylinos.cn>
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>

kernel/trace/trace_stack.c

@@ -472,7 +472,7 @@ static __init int stack_trace_init(void)
 			NULL, &stack_trace_fops);
 
 #ifdef CONFIG_DYNAMIC_FTRACE
-	trace_create_file("stack_trace_filter", 0444, d_tracer,
+	trace_create_file("stack_trace_filter", 0644, d_tracer,
 			  &trace_ops, &stack_trace_filter_fops);
 #endif