vhost: flush shadow Tx if no more packets

vhost: flush shadow Tx if no more packets

The current implementation of vhost_net in packed vring tries to fill
the shadow vector before send any actual changes to the guest. While
this can be beneficial for the throughput, it conflicts with some
bufferfloats methods like the linux kernel napi, that stops
transmitting packets if there are too much bytes/buffers in the
driver.

To solve it, we flush the shadow packets at the end of
virtio_dev_tx_packed if we have starved the vring, i.e. the next
buffer is not available for the device.

Since this last check can be expensive because of the atomic, we only
check it if we have not obtained the expected "count" packets. If it
happens to obtain "count" packets and there is no more available
packets the caller needs to keep call virtio_dev_tx_packed again.

Fixes: 31d6c6a5b820 ("vhost: optimize packed ring dequeue")
Cc: stable@dpdk.org
Signed-off-by: Eugenio Pérez <eperezma@redhat.com>
Reviewed-by: Maxime Coquelin <maxime.coquelin@redhat.com>