vhost: fix virtio-net header length with packed ring

vhost: fix virtio-net header length with packed ring

In case packed ring layout has been negotiated, but neither
Version 1 nor mergeable buffers, the Virtio-net header len
is assigned to the legacy devices value, which is wrong.

This patch fixes this with using the proper len as devices
using packed ring are not legacy devices.

Fixes: a922401f35cc ("vhost: add Rx support for packed ring")
Fixes: ae999ce49dcb ("vhost: add Tx support for packed ring")
Cc: stable@dpdk.org
Reported-by: Marvin Liu <yong.liu@intel.com>
Signed-off-by: Maxime Coquelin <maxime.coquelin@redhat.com>
Reviewed-by: Chenbo Xia <chenbo.xia@intel.com>