vhost: fix packed ring constants declaration

vhost: fix packed ring constants declaration

The packed ring defines were declared only if kernel
header does not declare them.
The problem is that they are not applied in upstream kernel,
and some changes in the names have been required.

This patch declares the defines unconditionally, which
fixes potential build issues.

Fixes: 297b1e7350f6 ("vhost: add virtio packed virtqueue defines")
Cc: stable@dpdk.org
Signed-off-by: Maxime Coquelin <maxime.coquelin@redhat.com>