um: Set parent-death signal for ubd io thread/process

The ubd io thread is not really a traditional thread. Set the
parent-death signal for it to ensure that it will be killed if
the UML kernel dies unexpectedly without proper cleanup.

Signed-off-by: Tiwei Bie <tiwei.btw@antgroup.com>
Link: https://patch.msgid.link/20241024142828.2612828-3-tiwei.btw@antgroup.com


Signed-off-by: Johannes Berg <johannes.berg@intel.com>