RDMA/mlx5: Fix memory leak in error flow for subscribe event routine

In case the second xa_insert() fails, the obj_event is not released.  Fix
the error unwind flow to free that memory to avoid a memory leak.

Fixes: 75973853 ("IB/mlx5: Enable subscription for device events over DEVX")
Link: https://lore.kernel.org/r/1647018361-18266-1-git-send-email-lyz_cs@pku.edu.cnSigned-off-by: Yongzhi Liu <lyz_cs@pku.edu.cn>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>

drivers/infiniband/hw/mlx5/devx.c

@@ -1886,8 +1886,10 @@ subscribe_event_xa_alloc(struct mlx5_devx_event_table *devx_event_table,
 				key_level2,
 				obj_event,
 				GFP_KERNEL);
-		if (err)
+		if (err) {
+			kfree(obj_event);
 			return err;
+		}
 		INIT_LIST_HEAD(&obj_event->obj_sub_list);
 	}