RCU: make the period of the idle timer adaptive

RCU: make the period of the idle timer adaptive

Basically, if the RCU idle timer, when (if!) it fires,
finds that the grace period isn't over, we increase the
timer's period (i.e., it will fire later, next time).
If, OTOH, it finds the grace period is already finished,
we decrease the timer's period (i.e., it will fire a bit
earlier next time).

The goal is to let the period timer sefl-adjust to a
number of 'misses', of the order of 1%.

Suggested-by: George Dunlap <george.dunlap@citrix.com>
Signed-off-by: Dario Faggioli <dario.faggioli@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>