Allocate io and request in single allocation

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

src/engine/engine_wt.c

@@ -167,7 +167,7 @@ int ocf_write_wt(struct ocf_request *req)
 	int lock = OCF_LOCK_NOT_ACQUIRED;
 	struct ocf_cache *cache = req->cache;
 
-	ocf_io_start(req->io);
+	ocf_io_start(&req->ioi.io);
 
 	/* Get OCF request - increase reference counter */
 	ocf_req_get(req);