Don't modify req->error for IOs outside io engines

`req->error` should be modified by the request's owner only; hence, if
the request has been allocated to handle IO (i.e. it's not an internal OCF
request) only engines can change it

Signed-off-by: Michal Mielewczyk <michal.mielewczyk@huawei.com>

src/metadata/metadata_raw.c

@@ -462,7 +462,6 @@ static void _raw_ram_flush_do_asynch_io_complete(ocf_cache_t cache,
 	OCF_DEBUG_MSG(cache, "Asynchronous flushing complete");
 
 	/* Call metadata flush completed call back */
-	ctx->req->error |= ctx->error;
 	ctx->complete(ctx->req, ctx->error);
 
 	env_free(ctx);

src/ocf_request.c

@@ -659,8 +659,6 @@ void ocf_forward_end(ocf_forward_token_t token, int error)
 {
 	struct ocf_request *req = ocf_req_forward_token_to_req(token);
 
-	req->error |= error;
-
 	if (token & 1) {
 		req->cache_error = req->cache_error ?: error;
 		ocf_req_forward_cache_put(req);