rcutorture: Check for unwatched readers

RCU is supposed to be watching all non-idle kernel code and also all
softirq handlers.  This commit adds some teeth to this statement by
adding a WARN_ON_ONCE().
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>

kernel/rcu/rcutorture.c

@@ -1377,6 +1377,7 @@ static bool rcu_torture_one_read(struct torture_random_state *trsp)
 	struct rt_read_seg *rtrsp1;
 	unsigned long long ts;
 
+	WARN_ON_ONCE(!rcu_is_watching());
 	newstate = rcutorture_extend_mask(readstate, trsp);
 	rcutorture_one_extend(&readstate, newstate, trsp, rtrsp++);
 	started = cur_ops->get_gp_seq();