rcu: Add synchronous grace-period waiting for RCU-tasks

It turns out to be easier to add the synchronous grace-period waiting
functions to RCU-tasks than to work around their absense in rcutorture,
so this commit adds them.  The key point is that the existence of
call_rcu_tasks() means that rcutorture needs an rcu_barrier_tasks().

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