alarmtimer: Remove dead return value in clock2alarm()

'clockid' can only be ALARM_REALTIME and ALARM_BOOTTIME. It's impossible to
return -1 and callers never check the return value.

Only alarm_clock_get_timespec(), alarm_clock_get_ktime(),
alarm_timer_create() and alarm_timer_nsleep() call clock2alarm(). These
callers use clockid_to_kclock() to get 'struct k_clock', which ensures
that clock2alarm() never returns -1.

Remove the impossible -1 return value, and add a warning to notify about any
future misuse of this function.

Signed-off-by: Su Hui <suhui@nfschina.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lore.kernel.org/all/20250430032734.2079290-3-suhui@nfschina.com

Author: Su Hui

kernel/time/alarmtimer.c

@@ -515,9 +515,9 @@ static enum alarmtimer_type clock2alarm(clockid_t clockid)
 {
 	if (clockid == CLOCK_REALTIME_ALARM)
 		return ALARM_REALTIME;
-	if (clockid == CLOCK_BOOTTIME_ALARM)
-		return ALARM_BOOTTIME;
-	return -1;
+
+	WARN_ON_ONCE(clockid != CLOCK_BOOTTIME_ALARM);
+	return ALARM_BOOTTIME;
 }
 
 /**