rcutorture: Expedited-GP batch progress access to torturing

This commit provides rcu_exp_batches_completed() and
rcu_exp_batches_completed_sched() functions to allow torture-test modules
to check how many expedited grace period batches have completed.
These are analogous to the existing rcu_batches_completed(),
rcu_batches_completed_bh(), and rcu_batches_completed_sched() functions.

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