y2038: alarm: fix half-second cut-off

Changing alarm_itimer accidentally broke the logic for arithmetic
rounding of half seconds in the return code.

Change it to a constant based on NSEC_PER_SEC, as suggested by
Ben Hutchings.

Fixes: bd40a17 ("y2038: itimer: change implementation to timespec64")
Reported-by: Ben Hutchings <[email protected]>
Signed-off-by: Arnd Bergmann <[email protected]>

Author: arndb

kernel/time/itimer.c

@@ -297,7 +297,7 @@ static unsigned int alarm_setitimer(unsigned int seconds)
 	 * better return too much than too little anyway
 	 */
 	if ((!it_old.it_value.tv_sec && it_old.it_value.tv_nsec) ||
-	      it_old.it_value.tv_nsec >= 500000)
+	      it_old.it_value.tv_nsec >= (NSEC_PER_SEC / 2))
 		it_old.it_value.tv_sec++;
 
 	return it_old.it_value.tv_sec;