um: Use os_set_pdeathsig helper in winch thread/process

Since we have a helper now, let's switch to using it. It will make
the code slightly more consistent.

Signed-off-by: Tiwei Bie <tiwei.btw@antgroup.com>
Link: https://patch.msgid.link/20241024142828.2612828-5-tiwei.btw@antgroup.com


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