fellow_cache: add result to fellow_cache_obj to latch errors

src/fellow_cache.c

@@ -714,7 +714,8 @@ struct fellow_cache_obj {
 	// ^^^ protected by mtx - do not add <64bit values
 
 	struct fellow_cache_lru		*lru;
-	uint64_t			ntouched;
+	struct fellow_cache_res		res;
+	uint8_t				ntouched;
 
 	struct buddy_ptr_extent		fco_mem;
 	struct buddy_ptr_page		fco_dowry;