virtio-net: unbreak csumed packets for XDP_PASS

We drop csumed packet when do XDP for packets. This breaks
XDP_PASS when GUEST_CSUM is supported. Fix this by allowing csum flag
to be set. With this patch, simple TCP works for XDP_PASS.

Cc: John Fastabend <john.r.fastabend@intel.com>
Signed-off-by: Jason Wang <jasowang@redhat.com>
Acked-by: John Fastabend <john.r.fastabend@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>