rcutorture: Print forward-progress test interval on error

This commit prints the duration of the forward-progress test interval in
the case that no forward progress was observed as an aid to debugging.
When forward progress does happen, it prints out the number of
rcu_torture_writer() versions and grace periods that elapsed during the
forward-progress test.  At the end of the run, it also prints the number
of attempted and actual forward-progress tests.

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