rtc: set the alarm to the next expiring timer

rtc: set the alarm to the next expiring timer

commit 74717b28cb32e1ad3c1042cafd76b264c8c0f68d upstream.

If there is any non expired timer in the queue, the RTC alarm is never set.
This is an issue when adding a timer that expires before the next non
expired timer.

Ensure the RTC alarm is set in that case.

Fixes: 2b2f5ff00f63 ("rtc: interface: ignore expired timers when enqueuing new timers")
Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>
[bwh: Backported to 3.2: open-code ktime_before()]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>