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>

Author: btw616

arch/um/drivers/chan_user.c

@@ -10,7 +10,6 @@
 #include <signal.h>
 #include <termios.h>
 #include <sys/ioctl.h>
-#include <sys/prctl.h>
 #include "chan_user.h"
 #include <os.h>
 #include <um_malloc.h>
@@ -162,7 +161,7 @@ static __noreturn int winch_thread(void *arg)
 	int count;
 	char c = 1;
 
-	prctl(PR_SET_PDEATHSIG, SIGKILL);
+	os_set_pdeathsig();
 
 	pty_fd = data->pty_fd;
 	pipe_fd = data->pipe_fd;