Traces of rcu_prep_idle events can be confusing because
rcu_cleanup_after_idle() does no tracing.  This commit therefore adds
this tracing.
Signed-off-by: NPaul E. McKenney <paul.mckenney@linaro.org>
Signed-off-by: NPaul E. McKenney <paulmck@linux.vnet.ibm.com>