Fix unnecessary page table activation

ostd/src/mm/page_table/node.rs

@@ -128,12 +128,12 @@ where
 
         let last_activated_paddr = current_page_table_paddr();
 
-        activate_page_table(self.raw, CachePolicy::Writeback);
-
         if last_activated_paddr == self.raw {
             return;
         }
 
+        activate_page_table(self.raw, CachePolicy::Writeback);
+
         // Increment the reference count of the current page table.
         self.inc_ref_count();