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: 32621ad7 ("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>