net/virtio: do not falsely claim to do IP checksum

The virtio driver is confused about the meaning of the ip_checksum
flag.  In DPDK, ip_checksum means the hardware is capable of checking
the Layer 3 IP checksum.  But KVM/QEMU does not do that. The flag
VIRTIO_NET_F_GUEST_CSUM controls whether the receive side does
Layer 4 (TCP/UDP) checksum offload.

Fix by erroring out any requests to do IP checksum.

Fixes: 96cb6711939e ("net/virtio: support Rx checksum offload")
Cc: stable@dpdk.org
Signed-off-by: Stephen Hemminger <stephen@networkplumber.org>
Acked-by: Yuanhan Liu <yuanhan.liu@linux.intel.com>