rcutorture: Add irqs-disabled test for call_rcu()

Mutation testing carried out by Iftekhar Ahmed of Oregon State
University showed that rcutorture is failing to test invocations
of call_rcu() having interrupts disabled.  This commit therefore
adds interrupt disabling around one of the existing invocations
of call_rcu() (and friends).

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
diff --git a/kernel/rcu/rcutorture.c b/kernel/rcu/rcutorture.c
index 633a68a..084a28a 100644
--- a/kernel/rcu/rcutorture.c
+++ b/kernel/rcu/rcutorture.c
@@ -1478,7 +1478,9 @@
 		 * The above smp_load_acquire() ensures barrier_phase load
 		 * is ordered before the folloiwng ->call().
 		 */
+		local_irq_disable(); /* Just to test no-irq call_rcu(). */
 		cur_ops->call(&rcu, rcu_torture_barrier_cbf);
+		local_irq_enable();
 		if (atomic_dec_and_test(&barrier_cbs_count))
 			wake_up(&barrier_wq);
 	} while (!torture_must_stop());