tracing/events: put TP_fast_assign into braces

Currently TP_fast_assign has a limitation that we can't define local
variables in it.

Here's one use case when we introduce __dynamic_array():

TP_fast_assign(
	type *p = __get_dynamic_array(item);

	foo(p);
	bar(p);
),

[ Impact: allow defining local variables in TP_fast_assign ]
Signed-off-by: NLi Zefan <lizf@cn.fujitsu.com>
LKML-Reference: <4A2384B1.90100@cn.fujitsu.com>
Signed-off-by: NSteven Rostedt <rostedt@goodmis.org>

include/trace/ftrace.h

@@ -471,7 +471,7 @@ static void ftrace_raw_event_##call(proto)				\
 		return;							\
 	entry	= ring_buffer_event_data(event);			\
 									\
-	assign;								\
+	{ assign; }							\
 									\
 	if (!filter_current_check_discard(event_call, entry, event))	\
 		trace_nowake_buffer_unlock_commit(event, irq_flags, pc); \