The open-coded computation of the used size doesn't take the event
into account when the VIRTIO_RING_F_EVENT_IDX feature is present.
Fix that by using vhost_get_used_size().

Fixes: 8ea8cf89 ("vhost: support event index")
Cc: stable@vger.kernel.org
Signed-off-by: NGreg Kurz <groug@kaod.org>
Link: https://lore.kernel.org/r/160171932300.284610.11846106312938909461.stgit@bahia.lanSigned-off-by: NMichael S. Tsirkin <mst@redhat.com>