vsock/virtio: change the maximum packet size allowed

Since now we are able to split packets, we can avoid limiting
their sizes to VIRTIO_VSOCK_DEFAULT_RX_BUF_SIZE.
Instead, we can use VIRTIO_VSOCK_MAX_PKT_BUF_SIZE as the max
packet size.

Signed-off-by: Stefano Garzarella <sgarzare@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Acked-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>