virtio_net: introduce virtnet_build_skb()

This logic is used in multiple places, now we separate it into
a helper.

Signed-off-by: Xuan Zhuo <xuanzhuo@linux.alibaba.com>
Acked-by: Jason Wang <jasowang@redhat.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>