docs: convert kref semaphore to mutex

instance, you have a list of items that are each kref-ed, and you wish

to get the first one.  You can't just pull the first item off the list

and kref_get() it.  That violates rule 3 because you are not already

holding a valid pointer.  You must add locks or semaphores.  For

instance:

holding a valid pointer.  You must add a mutex (or some other lock).

For instance:

static DECLARE_MUTEX(sem);

static DEFINE_MUTEX(mutex);

static LIST_HEAD(q);

struct my_data

{

static struct my_data *get_entry()

{

	struct my_data *entry = NULL;

	down(&sem);

	mutex_lock(&mutex);

	if (!list_empty(&q)) {

		entry = container_of(q.next, struct my_q_entry, link);

		kref_get(&entry->refcount);

	}

	up(&sem);

	mutex_unlock(&mutex);

	return entry;

}

static void put_entry(struct my_data *entry)

{

	down(&sem);

	mutex_lock(&mutex);

	kref_put(&entry->refcount, release_entry);

	up(&sem);

	mutex_unlock(&mutex);

}

The kref_put() return value is useful if you do not want to hold the

static void put_entry(struct my_data *entry)

{

	down(&sem);

	mutex_lock(&mutex);

	if (kref_put(&entry->refcount, release_entry)) {

		list_del(&entry->link);

		up(&sem);

		mutex_unlock(&mutex);

		kfree(entry);

	} else

		up(&sem);

		mutex_unlock(&mutex);

}

This is really more useful if you have to call other routines as part