Allocate io and request in single allocation

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

src/engine/engine_wi.c

@@ -140,7 +140,7 @@ int ocf_write_wi(struct ocf_request *req)
 
 	OCF_DEBUG_TRACE(req->cache);
 
-	ocf_io_start(req->io);
+	ocf_io_start(&req->ioi.io);
 
 	/* Get OCF request - increase reference counter */
 	ocf_req_get(req);