rcu/nocb: Move nocb field at the end of state struct

nocb_is_setup is a rarely used field, mostly on boot and CPU hotplug.
It shouldn't occupy the middle of the rcu state hot fields cacheline.

Move it to the end and build it conditionally while at it. More cold
NOCB fields are to come.
Signed-off-by: Frederic Weisbecker <frederic@kernel.org>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Reviewed-by: Paul E. McKenney <paulmck@kernel.org>
Signed-off-by: Neeraj Upadhyay <neeraj.upadhyay@kernel.org>

kernel/rcu/tree.h

@@ -411,7 +411,6 @@ struct rcu_state {
 	arch_spinlock_t ofl_lock ____cacheline_internodealigned_in_smp;
 						/* Synchronize offline with */
 						/*  GP pre-initialization. */
-	int nocb_is_setup;			/* nocb is setup from boot */
 
 	/* synchronize_rcu() part. */
 	struct llist_head srs_next;	/* request a GP users. */
@@ -420,6 +419,10 @@ struct rcu_state {
 	struct sr_wait_node srs_wait_nodes[SR_NORMAL_GP_WAIT_HEAD_MAX];
 	struct work_struct srs_cleanup_work;
 	atomic_t srs_cleanups_pending; /* srs inflight worker cleanups. */
+
+#ifdef CONFIG_RCU_NOCB_CPU
+	int nocb_is_setup;			/* nocb is setup from boot */
+#endif
 };
 
 /* Values for rcu_state structure's gp_flags field. */