Hopefully final fix of Bug#11766591 59733: Possible deadlock

when buffered changes are to be discarded

ibuf_add_free_page(): Lower the latching order of the newly allocated page
to SYNC_IBUF_TREE_NODE_NEW after latching the insert buffer tree root.

This bug always was bogus UNIV_SYNC_DEBUG alarm. The function
buf_block_dbg_add_level() is a no-op unless UNIV_SYNC_DEBUG is defined.

storage/innobase/ibuf/ibuf0ibuf.c

@@ -1687,12 +1687,12 @@ ibuf_add_free_page(
 
 	mutex_enter(&ibuf_mutex);
 
+	root = ibuf_tree_root_get(ibuf_data, space, &mtr);
+
 #ifdef UNIV_SYNC_DEBUG
 	buf_page_dbg_add_level(page, SYNC_TREE_NODE_NEW);
 #endif /* UNIV_SYNC_DEBUG */
 
-	root = ibuf_tree_root_get(ibuf_data, space, &mtr);
-
 	/* Add the page to the free list and update the ibuf size data */
 
 	flst_add_last(root + PAGE_HEADER + PAGE_BTR_IBUF_FREE_LIST,

storage/innodb_plugin/ibuf/ibuf0ibuf.c

@@ -1770,13 +1770,13 @@ ibuf_add_free_page(void)
 
 		mutex_enter(&ibuf_mutex);
 
+		root = ibuf_tree_root_get(&mtr);
+
 		buf_block_dbg_add_level(block, SYNC_IBUF_TREE_NODE_NEW);
 
 		page = buf_block_get_frame(block);
 	}
 
-	root = ibuf_tree_root_get(&mtr);
-
 	/* Add the page to the free list and update the ibuf size data */
 
 	flst_add_last(root + PAGE_HEADER + PAGE_BTR_IBUF_FREE_LIST,