rcutorture: Reduce priority of forward-progress testing

On !SMP tests, the forward-progress kthread might prevent RCU's
grace-period kthread from running, which would defeat RCU's
forward-progress measures.  On PREEMPT tests without RCU priority
boosting, the forward-progress kthread might preempt a reader for an
extended time period, which would also defeat RCU's forward-progress
measures.  This commit therefore reduced rcutorture's forward-progress
kthread's priority in those cases.

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