RDMA/core: Use refcount_t instead of atomic_t on refcount of mcast_member

The refcount_t API will WARN on underflow and overflow of a reference
counter, and avoid use-after-free risks.

Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Weihang Li <[e-mail]>
Signed-off-by: Jason Gunthorpe <[e-mail]>

drivers/infiniband/core/multicast.c

···
 	struct mcast_group	*group;
 	struct list_head	list;
 	enum mcast_state	state;
-	atomic_t		refcount;
+	refcount_t		refcount;
 	struct completion	comp;
 };
 
···
 
 static void deref_member(struct mcast_member *member)
 {
-	if (atomic_dec_and_test(&member->refcount))
+	if (refcount_dec_and_test(&member->refcount))
 		complete(&member->comp);
 }
 
···
 	while (!list_empty(&group->active_list)) {
 		member = list_entry(group->active_list.next,
 				    struct mcast_member, list);
-		atomic_inc(&member->refcount);
+		refcount_inc(&member->refcount);
 		list_del_init(&member->list);
 		adjust_membership(group, member->multicast.rec.join_state, -1);
 		member->state = MCAST_ERROR;
···
 				    struct mcast_member, list);
 		multicast = &member->multicast;
 		join_state = multicast->rec.join_state;
-		atomic_inc(&member->refcount);
+		refcount_inc(&member->refcount);
 
 		if (join_state == (group->rec.join_state & join_state)) {
 			status = cmp_rec(&group->rec, &multicast->rec,
···
 	member = list_entry(group->pending_list.next,
 			    struct mcast_member, list);
 	if (group->last_join == member) {
-		atomic_inc(&member->refcount);
+		refcount_inc(&member->refcount);
 		list_del_init(&member->list);
 		spin_unlock_irq(&group->lock);
 		ret = member->multicast.callback(status, &member->multicast);
···
 	member->multicast.callback = callback;
 	member->multicast.context = context;
 	init_completion(&member->comp);
-	atomic_set(&member->refcount, 1);
+	refcount_set(&member->refcount, 1);
 	member->state = MCAST_JOINING;
 
 	member->group = acquire_group(&dev->port[port_num - dev->start_port],