virtio_net: introduce receive_small_build_xdp

Simplifying receive_small() function. Bringing the logic relating to
build_skb together.

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