ftrace: Unify effect of writing to trace_options and option/*

"echo noglobal-clock > trace_options" can be used to change trace
clock but "echo 0 > options/global-clock" can't. The flag toggling
will be silently accepted without actually changing the clock callback.

We can fix it by using set_tracer_flags() in
trace_options_core_write().

Changelog:
v1->v2: Simplified switch() after Li Zefan <lizf@cn.fujitsu.com>'s
        suggestion

Signed-off-by: Zhao Lei <zhaolei@cn.fujitsu.com>
Cc: Steven Rostedt <srostedt@redhat.com>
Cc: Li Zefan <lizf@cn.fujitsu.com>
Signed-off-by: Frederic Weisbecker <fweisbec@gmail.com>

kernel/trace/trace.c

@@ -3896,17 +3896,9 @@ trace_options_core_write(struct file *filp, const char __user *ubuf, size_t cnt,
 	if (ret < 0)
 		return ret;
 
-	switch (val) {
-	case 0:
-		trace_flags &= ~(1 << index);
-		break;
-	case 1:
-		trace_flags |= 1 << index;
-		break;
-
-	default:
+	if (val != 0 && val != 1)
 		return -EINVAL;
-	}
+	set_tracer_flags(1 << index, val);
 
 	*ppos += cnt;