net/virtio: revert not claiming IP checksum offload

net/virtio: revert not claiming IP checksum offload

This reverts
commit 4dab342b7522 ("net/virtio: do not falsely claim to do IP checksum").

The description of rxmode->hw_ip_checksum is:

     hw_ip_checksum   : 1, /**< IP/UDP/TCP checksum offload enable. */

Despite its name, this field can be set by an application to enable L3
and L4 checksums. In case of virtio, only L4 checksum is supported and
L3 checksums flags will always be set to "unknown".

Fixes: 4dab342b7522 ("net/virtio: do not falsely claim to do IP checksum")
Cc: stable@dpdk.org
Signed-off-by: Olivier Matz <olivier.matz@6wind.com>
Acked-by: Yuanhan Liu <yliu@fridaylinux.org>