Fix discard step callback refcount

Signed-off-by: Rafal Stefanowski <rafal.stefanowski@huawei.com>
Signed-off-by: Michal Mielewczyk <michal.mielewczyk@huawei.com>

src/engine/engine_discard.c

@@ -156,6 +156,8 @@ static int _ocf_discard_step_do(struct ocf_request *req)
 		ocf_hb_req_prot_unlock_wr(req);
 
 		if (req->info.flush_metadata) {
+			env_atomic_inc(&req->req_remaining);
+
 			/* Request was dirty and need to flush metadata */
 			ocf_metadata_flush_do_asynch(cache, req,
 					_ocf_discard_step_complete);