rcu/nocb: Remove obsolete nocb_gp_head and nocb_gp_tail fields

	struct timer_list nocb_timer;	/* Enforce finite deferral. */

	/* The following fields are used by GP kthread, hence own cacheline. */

	struct rcu_head *nocb_gp_head ____cacheline_internodealigned_in_smp;

					/* CBs waiting for GP. */

	struct rcu_head **nocb_gp_tail;

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

	bool nocb_gp_sleep ____cacheline_internodealigned_in_smp;

					/* Is the nocb GP thread asleep? */

	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;