rcutorture: Change units of onoff_interval to jiffies

Some RCU bugs have been sensitive to the frequency of CPU-hotplug
operations, which have been gradually increased over time.  But this
frequency is now at the one-second lower limit that can be specified using
the rcutorture.onoff_interval kernel parameter.  This commit therefore
changes the units of rcutorture.onoff_interval from seconds to jiffies,
and also sets the value specified for this kernel parameter in the TREE03
rcutorture scenario to 200, which is 200 milliseconds for HZ=1000.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>