rcu/nocb: Remove obsolete nocb_head and nocb_tail fields

	/* 5) Callback offloading. */

#ifdef CONFIG_RCU_NOCB_CPU

	struct rcu_head *nocb_head;	/* CBs waiting for kthread. */

	struct rcu_head **nocb_tail;

	atomic_long_t nocb_q_count;	/* # CBs waiting for nocb */

	atomic_long_t nocb_q_count_lazy; /*  invocation (all stages). */

	struct rcu_head *nocb_cb_head;	/* CBs ready to invoke. */

					/* CBs waiting for GP. */

	struct rcu_head **nocb_gp_tail;

	bool nocb_gp_sleep;		/* Is the nocb GP thread asleep? */

	bool nocb_gp_forced;		/* Forced nocb GP thread wakeup? */

	struct swait_queue_head nocb_gp_wq; /* For nocb kthreads to sleep on. */

	bool nocb_cb_sleep;		/* Is the nocb CB thread asleep? */

	struct task_struct *nocb_cb_kthread;

/* Initialize per-rcu_data variables for no-CBs CPUs. */

static void __init rcu_boot_init_nocb_percpu_data(struct rcu_data *rdp)

{

	rdp->nocb_tail = &rdp->nocb_head;

	init_swait_queue_head(&rdp->nocb_cb_wq);

	init_swait_queue_head(&rdp->nocb_gp_wq);

	rdp->nocb_cb_tail = &rdp->nocb_cb_head;