alarmtimer: Use EOPNOTSUPP instead of ENOTSUPP

commit f18ddc1 upstream.

ENOTSUPP is not supposed to be returned to userspace. This was found on an
OpenPower machine, where the RTC does not support set_alarm.

On that system, a clock_nanosleep(CLOCK_REALTIME_ALARM, ...) results in
"524 Unknown error 524"

Replace it with EOPNOTSUPP which results in the expected "95 Operation not
supported" error.

Fixes: 1c6b39a (alarmtimers: Return -ENOTSUPP if no RTC device is present)
Signed-off-by: Thadeu Lima de Souza Cascardo <cascardo@canonical.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: stable@vger.kernel.org
Link: https://lkml.kernel.org/r/20190903171802.28314-1-cascardo@canonical.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

kernel/time/alarmtimer.c

@@ -544,7 +544,7 @@ static int alarm_timer_create(struct k_itimer *new_timer)
 	enum  alarmtimer_type type;
 
 	if (!alarmtimer_get_rtcdev())
-		return -ENOTSUPP;
+		return -EOPNOTSUPP;
 
 	if (!capable(CAP_WAKE_ALARM))
 		return -EPERM;
@@ -772,7 +772,7 @@ static int alarm_timer_nsleep(const clockid_t which_clock, int flags,
 	struct restart_block *restart;
 
 	if (!alarmtimer_get_rtcdev())
-		return -ENOTSUPP;
+		return -EOPNOTSUPP;
 
 	if (flags & ~TIMER_ABSTIME)
 		return -EINVAL;