optimize rbtree initialization and insert
diff --git a/src/core/ngx_open_file_cache.c b/src/core/ngx_open_file_cache.c
index dab98b3..d518f4b 100644
--- a/src/core/ngx_open_file_cache.c
+++ b/src/core/ngx_open_file_cache.c
@@ -53,11 +53,8 @@
         return NULL;
     }
 
-    ngx_rbtree_sentinel_init(sentinel);
-
-    cache->rbtree.root = sentinel;
-    cache->rbtree.sentinel = sentinel;
-    cache->rbtree.insert = ngx_open_file_cache_rbtree_insert_value;
+    ngx_rbtree_init(&cache->rbtree, sentinel,
+                    ngx_open_file_cache_rbtree_insert_value);
 
     cache->current = 0;
     cache->max = max;