virtio_net: replace netdev_alloc_skb_ip_align() with napi_alloc_skb()

This gives small but noticeable rx performance improvement (2-3%)
and will allow exploiting future napi improvement.

Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>