rcutorture: Add random real-time preemption

rcutorture: Add random real-time preemption

This commit adds the rcutorture.preempt_duration kernel module parameter,
which gives the real-time preemption duration in milliseconds (zero to
disable, which is the default) and also the rcutorture.preempt_interval
module parameter, which gives the interval between successive preemptions,
also in milliseconds, defaulting to one second.  The CPU to preempt is
chosen at random from those online at that time.  Races between preempting
a given CPU and that CPU going offline are ignored, and preemption is
forgone when this occurs.

Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Signed-off-by: Uladzislau Rezki (Sony) <urezki@gmail.com>