rcutorture: Add batch number to script printout

Currently, the scripts print "----Start batch" at the beginning of each
batch, which does serve as a good visual delimiter between batches.
Unfortunately, if there are a lot of batches, it is hard to quickly
estimate test runtime from the output of "--dryrun sched".  This commit
therefore adds a batch number, so that the beginning-of-batch output
looks like this "----Start batch 10" for the tenth batch.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: Josh Triplett <josh@joshtriplett.org>