rtc: rtc_read_alarm() handles wraparound

While 0e36a9a4 ("rtc: fix readback from
/sys/class/rtc/rtc?/wakealarm") made sure that active alarms were never
returned with invalid "wildcard" fields (negative), it can still report
(wrongly) that the alarm triggers in the past.

Example, if it's now 10am, an alarm firing at 5am will be triggered
TOMORROW not today.  (Which may also be next month or next year...)

This updates that alarm handling in three ways:

  * Handle alarm rollover in the common cases of RTCs that don't
    support matching on all date fields.

  * Skip the invalid-field logic when it's not needed.

  * Minor bugfix ... tm_isdst should be ignored, it's one of the
    fields Linux doesn't maintain.

A warning is emitted for some of the unhandled rollover cases, but the
possible combinations are a bit too numerous to handle every bit of
potential hardware and firmware braindamage.

Signed-off-by: David Brownell <dbrownell@users.sourceforge.net>
Cc: Mark Lord <lkml@rtr.ca>
Acked-by: Alessandro Zummo <a.zummo@towertech.it>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>