rcutorture: Add forward-progress tests for RCU grace periods

This commit adds a kthread that loops going into and out of RCU
read-side critical sections, but also including a cond_resched(),
optionally guarded by a check of need_resched(), in that same loop.
This commit relies solely on rcu_torture_writer() progress to judge
the forward progress of grace periods.

Note that Tasks RCU and SRCU are exempted from forward-progress testing
due their (intentionally) less-robust forward-progress guarantees.

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

kernel/rcu/update.c

@@ -472,6 +472,7 @@ int rcu_jiffies_till_stall_check(void)
 	}
 	return till_stall_check * HZ + RCU_STALL_DELAY_DELTA;
 }
+EXPORT_SYMBOL_GPL(rcu_jiffies_till_stall_check);
 
 void rcu_sysrq_start(void)
 {