um: ubd: Initialize ubd's disk pointer in ubd_add

Currently, the initialization of the disk pointer in the ubd structure
is missing. It should be initialized with the allocated gendisk pointer
in ubd_add().

Fixes: 32621ad ("ubd: remove the ubd_gendisk array")
Signed-off-by: Tiwei Bie <tiwei.btw@antgroup.com>
Acked-By: Anton Ivanov <anton.ivanov@cambridgegreys.com>
Link: https://patch.msgid.link/20241104163203.435515-2-tiwei.btw@antgroup.com
Signed-off-by: Johannes Berg <johannes.berg@intel.com>

arch/um/drivers/ubd_kern.c

@@ -898,6 +898,8 @@ static int ubd_add(int n, char **error_out)
 	if (err)
 		goto out_cleanup_disk;
 
+	ubd_dev->disk = disk;
+
 	return 0;
 
 out_cleanup_disk: