BACKPORT: time: tick-sched: Split tick_nohz_stop_sched_tick()

In order to address the issue with short idle duration predictions
by the idle governor after the scheduler tick has been stopped, split
tick_nohz_stop_sched_tick() into two separate routines, one computing
the time to the next timer event and the other simply stopping the
tick when the time to the next timer event is known.

Prepare these two routines to be called separately, as one of them
will be called by the idle governor in the cpuidle_select() code
path after subsequent changes.

Update the former callers of tick_nohz_stop_sched_tick() to use
the new routines, tick_nohz_next_event() and tick_nohz_stop_tick(),
instead of it and move the updates of the sleep_length field in
struct tick_sched into __tick_nohz_idle_stop_tick() as it doesn't
need to be updated anywhere else.

There should be no intentional visible changes in functionality
resulting from this change.

Cherry-picked from 23a8d888107ce4ce444eab2dcebf4cfb3578770b

 - Fixed conflict with tick_nohz_stopped_cpu not appearing in the
   chunk context. Trivial fix.

Change-Id: I43eb34c4799b4b9f18842e047c3d016394c22743
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Leo Yan <leo.yan@linaro.org>
Cc: Todd Kjos <tkjos@google.com>
Cc: Joel Fernandes <joelaf@google.com>
Cc: Paven Kondati <pkondeti@qti.qualcomm.com>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>