virtio-net: XDP support for small buffers

virtio-net: XDP support for small buffers

Commit 11c1dea1f3b7 ("virtio_net: Add XDP support") leaves the case of
small receive buffer untouched. This will confuse the user who want to
set XDP but use small buffers. Other than forbid XDP in small buffer
mode, let's make it work. XDP then can only work at skb->data since
virtio-net create skbs during refill, this is sub optimal which could
be optimized in the future.

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>