vsock: prevent transport modules unloading

This patch adds 'module' member in the 'struct vsock_transport'
in order to get/put the transport module. This prevents the
module unloading while sockets are assigned to it.

We increase the module refcnt when a socket is assigned to a
transport, and we decrease the module refcnt when the socket
is destructed.

Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Jorgen Hansen <jhansen@vmware.com>
Signed-off-by: Stefano Garzarella <sgarzare@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/vmw_vsock/virtio_transport.c

@@ -462,6 +462,8 @@ static void virtio_vsock_rx_done(struct virtqueue *vq)
 
 static struct virtio_transport virtio_transport = {
 	.transport = {
+		.module                   = THIS_MODULE,
+
 		.get_local_cid            = virtio_transport_get_local_cid,
 
 		.init                     = virtio_transport_do_socket_init,