net/virtio: fix in-order Tx path for split ring

net/virtio: fix in-order Tx path for split ring

When IN_ORDER feature is negotiated, device may just write out a
single used ring entry for a batch of buffers:

"""
Some devices always use descriptors in the same order in which they
have been made available. These devices can offer the VIRTIO_F_IN_ORDER
feature. If negotiated, this knowledge allows devices to notify the
use of a batch of buffers to the driver by only writing out a single
used ring entry with the id corresponding to the head entry of the
descriptor chain describing the last buffer in the batch.

The device then skips forward in the ring according to the size of
the batch. Accordingly, it increments the used idx by the size of
the batch.

The driver needs to look up the used id and calculate the batch size
to be able to advance to where the next used ring entry will be written
by the device.
"""

Currently, the in-order Tx path in split ring can't handle this.
With this patch, driver will allocate desc_extra[] based on the
index in avail/used ring instead of the index in descriptor table.
And driver can just relay on the used->idx written by device to
reclaim the descriptors and Tx buffers.

Fixes: e5f456a98d3c ("net/virtio: support in-order Rx and Tx")
Cc: stable@dpdk.org
Signed-off-by: Tiwei Bie <tiwei.bie@intel.com>
Reviewed-by: Maxime Coquelin <maxime.coquelin@redhat.com>