IB/mlx5: Add missing error code

Set err to -ENOMEM if kzalloc fails instead of 0.

Fixes: 7597385 ("IB/mlx5: Enable subscription for device events over DEVX")
Link: https://lore.kernel.org/r/20210222122343.19720-1-yuehaibing@huawei.com
Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Acked-by: Leon Romanovsky <leonro@nvidia.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>

drivers/infiniband/hw/mlx5/devx.c

@@ -2073,8 +2073,10 @@ static int UVERBS_HANDLER(MLX5_IB_METHOD_DEVX_SUBSCRIBE_EVENT)(
 
 		num_alloc_xa_entries++;
 		event_sub = kzalloc(sizeof(*event_sub), GFP_KERNEL);
-		if (!event_sub)
+		if (!event_sub) {
+			err = -ENOMEM;
 			goto err;
+		}
 
 		list_add_tail(&event_sub->event_list, &sub_list);
 		uverbs_uobject_get(&ev_file->uobj);