rcutorture: Emulate dyntick aspect of userspace nohz_full sojourn

During an actual call_rcu() flood, there would be frequent trips to
userspace (in-kernel call_rcu() floods must be otherwise housebroken).
Userspace execution on nohz_full CPUs implies an RCU dyntick idle/not-idle
transition pair, so this commit adds emulation of that pair.

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

kernel/rcu/rcutorture.c

@@ -1761,6 +1761,11 @@ static unsigned long rcu_torture_fwd_prog_cbfree(void)
 		kfree(rfcp);
 		freed++;
 		rcu_torture_fwd_prog_cond_resched(freed);
+		if (tick_nohz_full_enabled()) {
+			local_irq_save(flags);
+			rcu_momentary_dyntick_idle();
+			local_irq_restore(flags);
+		}
 	}
 	return freed;
 }
@@ -1835,6 +1840,7 @@ static void rcu_torture_fwd_prog_nr(int *tested, int *tested_tries)
 static void rcu_torture_fwd_prog_cr(void)
 {
 	unsigned long cver;
+	unsigned long flags;
 	unsigned long gps;
 	int i;
 	long n_launders;
@@ -1894,6 +1900,11 @@ static void rcu_torture_fwd_prog_cr(void)
 		}
 		cur_ops->call(&rfcp->rh, rcu_torture_fwd_cb_cr);
 		rcu_torture_fwd_prog_cond_resched(n_launders + n_max_cbs);
+		if (tick_nohz_full_enabled()) {
+			local_irq_save(flags);
+			rcu_momentary_dyntick_idle();
+			local_irq_restore(flags);
+		}
 	}
 	stoppedat = jiffies;
 	n_launders_cb_snap = READ_ONCE(n_launders_cb);