rcutorture: Add end-of-test check to rcu_torture_fwd_prog() loop

The second and subsequent forward-progress kthreads loop waiting for
the first forward-progress kthread to start the next test interval.
Unfortunately, if the test ends while one of those kthreads is waiting,
the test will hang.  This hang occurs because that wait loop fails to
check for the end of the test.  This commit therefore adds an end-of-test
check to that wait loop.

Signed-off-by: Paul E. McKenney <paulmck@kernel.org>