net/packet: fix packet drop as of virtio gso

When we use raw socket as the vhost backend, a packet from virito with
gso offloading information, cannot be sent out in later validaton at
xmit path, as we did not set correct skb->protocol which is further used
for looking up the gso function.

To fix this, we set this field according to virito hdr information.

Fixes: e858fae2 ("virtio_net: use common code for virtio_net_hdr and skb GSO conversion")
Signed-off-by: Jianfeng Tan <jianfeng.tan@linux.alibaba.com>
Signed-off-by: David S. Miller <davem@davemloft.net>