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 <[email protected]>
Link: https://patch.msgid.link/[email protected]
Signed-off-by: Johannes Berg <[email protected]>

Author: btw616

arch/um/drivers/ubd_kern.c

@@ -1499,6 +1499,7 @@ int io_thread(void *arg)
 {
 	int n, count, written, res;
 
+	os_set_pdeathsig();
 	os_fix_helper_signals();
 
 	while(1){