rcu: Precompute RCU_FAST_NO_HZ timer offsets
Ported from OSS commit:
    aa9b163

    When a CPU is entering dyntick-idle mode, tick_nohz_stop_sched_tick()
    calls rcu_needs_cpu() see if RCU needs that CPU, and, if not, computes the
    next wakeup time based on the timer wheels.  Only later, when actually
    entering the idle loop, rcu_prepare_for_idle() will be invoked.  In some
    cases, rcu_prepare_for_idle() will post timers to wake the CPU back up.
    But all for naught: The next wakeup time for the CPU has already been
    computed, and posting a timer afterwards does not force that wakeup
    time to be recomputed.  This means that rcu_prepare_for_idle()'s have
    no effect.

    This is not a problem on a busy system because something else will wake
    up the CPU soon enough.  However, on lightly loaded systems, the CPU
    might stay asleep for a considerable length of time.  If that CPU has
    a callback that the rest of the system is waiting on, the system might
    run very slowly or (in theory) even hang.

    This commit avoids this problem by having rcu_needs_cpu() give
    tick_nohz_stop_sched_tick() an estimate of when RCU will need the CPU
    to wake back up, which tick_nohz_stop_sched_tick() takes into account
    when programming the CPU's wakeup time.  An alternative approach is
    for rcu_prepare_for_idle() to use hrtimers instead of normal timers,
    but timers are much more efficient than are hrtimers for frequently
    and repeatedly posting and cancelling a given timer, which is exactly
    what RCU_FAST_NO_HZ does.