ntp: Make sure RTC is synchronized when time goes backwards

ntp: Make sure RTC is synchronized when time goes backwards

sync_hw_clock() is normally called every 11 minutes when time is
synchronized. This issue is that this periodic timer uses the REALTIME
clock, so when time moves backwards (the NTP server jumps into the past),
the timer expires late.

If the timer expires late, which can be days later, the RTC will no longer
be updated, which is an issue if the device is abruptly powered OFF during
this period. When the device will restart (when powered ON), it will have
the date prior to the ADJ_SETOFFSET call.

A normal NTP server should not jump in the past like that, but it is
possible... Another way of reproducing this issue is to use phc2sys to
synchronize the REALTIME clock with, for example, an IRIG timecode with
the source always starting at the same date (not synchronized).

Also, if the time jump in the future by less than 11 minutes, the RTC may
not be updated immediately (minor issue). Consider the following scenario:
 - Time is synchronized, and sync_hw_clock() was just called (the timer
   expires in 11 minutes).
 - A time jump is realized in the future by a couple of minutes.
 - The time is synchronized again.
 - Users may expect that RTC to be updated as soon as possible, and not
   after 11 minutes (for the same reason, if a power loss occurs in this
   period).

Cancel periodic timer on any time jump (ADJ_SETOFFSET) greater than or
equal to 1s. The timer will be relaunched at the end of do_adjtimex() if
NTP is still considered synced. Otherwise the timer will be relaunched
later when NTP is synced. This way, when the time is synchronized again,
the RTC is updated after less than 2 seconds.

Signed-off-by: Benjamin ROBIN <dev@benjarobin.fr>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lore.kernel.org/all/20240908140836.203911-1-dev@benjarobin.fr