rcutorture: Add three-level tree test for Tree SRCU

This commit adds a test for a three-level srcu_node tree for Tree SRCU
in the existing SRCU-P scenario.  This requires enabling CONFIG_RCU_EXPERT,
so the CONFIG_RCU_EXPERT=n scenario is now SRCU-N.  The reason for using
SRCU-P for the tall tree is that preemption raises the possibility of
locating more bugs than does the non-preemptive SRCU-N.
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

tools/testing/selftests/rcutorture/configs/rcu/SRCU-N

@@ -5,4 +5,4 @@ CONFIG_HOTPLUG_CPU=y
 CONFIG_PREEMPT_NONE=y
 CONFIG_PREEMPT_VOLUNTARY=n
 CONFIG_PREEMPT=n
-CONFIG_RCU_EXPERT=y
+#CHECK#CONFIG_RCU_EXPERT=n

tools/testing/selftests/rcutorture/configs/rcu/SRCU-P

@@ -2,9 +2,11 @@ CONFIG_RCU_TRACE=n
 CONFIG_SMP=y
 CONFIG_NR_CPUS=8
 CONFIG_HOTPLUG_CPU=y
+CONFIG_RCU_EXPERT=y
+CONFIG_RCU_FANOUT=2
+CONFIG_RCU_FANOUT_LEAF=2
 CONFIG_PREEMPT_NONE=n
 CONFIG_PREEMPT_VOLUNTARY=n
 CONFIG_PREEMPT=y
-#CHECK#CONFIG_RCU_EXPERT=n
 CONFIG_DEBUG_LOCK_ALLOC=y
 CONFIG_PROVE_LOCKING=y