rcu: Track CPU-hotplug duration statistics

Many rcutorture runs include CPU-hotplug operations in their stress
testing.  This commit accumulates statistics on the durations of these
operations in deference to the recent concern about the overhead and
latency of these operations.

Signed-off-by: Paul E. McKenney <paul.mckenney@linaro.org>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: Josh Triplett <josh@joshtriplett.org>