tracing/stat: remember to free root node

When closing a trace_stat file, we destroy the rbtree constructed during
file open, but there is memory leak that the root node is not freed.

[ Impact: fix memory leak when closing a trace_stat file ]
Signed-off-by: NLi Zefan <lizf@cn.fujitsu.com>
Signed-off-by: NFrederic Weisbecker <fweisbec@gmail.com>

kernel/trace/trace_stat.c

@@ -60,8 +60,8 @@ static struct rb_node *release_next(struct rb_node *node)
 		return node->rb_right;
 	else {
 		if (!parent)
-			return NULL;
-		if (parent->rb_left == node)
+			;
+		else if (parent->rb_left == node)
 			parent->rb_left = NULL;
 		else
 			parent->rb_right = NULL;