notifiers: __rcu annotations

struct notifier_block {

	int (*notifier_call)(struct notifier_block *, unsigned long, void *);

	struct notifier_block *next;

	struct notifier_block __rcu *next;

	int priority;

};

struct atomic_notifier_head {

	spinlock_t lock;

	struct notifier_block *head;

	struct notifier_block __rcu *head;

};

struct blocking_notifier_head {

	struct rw_semaphore rwsem;

	struct notifier_block *head;

	struct notifier_block __rcu *head;

};

struct raw_notifier_head {

	struct notifier_block *head;

	struct notifier_block __rcu *head;

};

struct srcu_notifier_head {

	struct mutex mutex;

	struct srcu_struct srcu;

	struct notifier_block *head;

	struct notifier_block __rcu *head;

};

#define ATOMIC_INIT_NOTIFIER_HEAD(name) do {	\