Unify core naming convention (core_obj -> core)

Signed-off-by: Robert Baldyga <robert.baldyga@intel.com>

src/engine/engine_zero.c

@@ -50,7 +50,7 @@ static const struct ocf_io_if _io_if_zero_purge = {
 static void _ocf_zero_io_flush_metadata(struct ocf_request *req, int error)
 {
 	if (error) {
-		env_atomic_inc(&req->cache->core_obj[req->core_id].counters->
+		env_atomic_inc(&req->cache->core[req->core_id].counters->
 				cache_errors.write);
 		req->error = error;
 	}