net/virtio: fix Tx checksum for tunnel packets

net/virtio: fix Tx checksum for tunnel packets

Tx prepare method calls rte_net_intel_cksum_prepare(), which
handles tunnel packets correctly, but Tx burst path does not
take tunnel presence into account when computing the offsets.

Fixes: 58169a9c8153 ("net/virtio: support Tx checksum offload")
Cc: stable@dpdk.org
Signed-off-by: Ivan Malov <ivan.malov@oktetlabs.ru>
Reviewed-by: Andrew Rybchenko <andrew.rybchenko@oktetlabs.ru>
Reviewed-by: Chenbo Xia <chenbo.xia@intel.com>
Reviewed-by: Olivier Matz <olivier.matz@6wind.com>