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

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_device	*dev;
 	spinlock_t		lock;
 	struct rb_root		table;
-	atomic_t		refcount;
+	refcount_t		refcount;
 	struct completion	comp;
 	u32			port_num;
 };
···
 
 static void deref_port(struct mcast_port *port)
 {
-	if (atomic_dec_and_test(&port->refcount))
+	if (refcount_dec_and_test(&port->refcount))
 		complete(&port->comp);
 }
 
···
 		kfree(group);
 		group = cur_group;
 	} else
-		atomic_inc(&port->refcount);
+		refcount_inc(&port->refcount);
 found:
 	atomic_inc(&group->refcount);
 	spin_unlock_irqrestore(&port->lock, flags);
···
 		spin_lock_init(&port->lock);
 		port->table = RB_ROOT;
 		init_completion(&port->comp);
-		atomic_set(&port->refcount, 1);
+		refcount_set(&port->refcount, 1);
 		++count;
 	}