virtio_ring: Fix potential mem leak in virtqueue_add_indirect_packed

'desc' should be freed before leaving from err handing path.

Fixes: 1ce9e605 ("virtio_ring: introduce packed ring support")
Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Acked-by: Jason Wang <jasowang@redhat.com>
stable@vger.kernel.org