alarmtimer: Switch spin_{lock,unlock}_irqsave() to guards

Using guard/scoped_guard() to simplify code. Using guard() to remove
'goto unlock' label is neater especially.

[ tglx: Brought back the scoped_guard()'s which were dropped in v2 and
  	simplified alarmtimer_rtc_add_device() ]

Signed-off-by: Su Hui <suhui@nfschina.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Acked-by: John Stultz <jstultz@google.com>
Link: https://lore.kernel.org/all/20250430032734.2079290-4-suhui@nfschina.com