rcu: Distinguish between boosting and boosted

RCU priority boosting's tracing did not distinguish between ongoing
boosting and completion of boosting.  This commit therefore adds this
capability.

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