rcu: Provide rcu_batches_completed_sched() for TINY_RCU

A bug in rcutorture has caused it to ignore completed batches.
In preparation for fixing that bug, this commit provides TINY_RCU with
the required rcu_batches_completed_sched().

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