mempool: format large block header before freeing it

Pages allocated with alloc_page() have no metadata, so we must format the
header before returing it to the free range tree.

mempool.cc

@@ -196,6 +196,7 @@ void* alloc_page()
 
 void free_page(void* v)
 {
+    new (v) page_range(page_size);
     free_large(v + page_size);
 }