virtio-net: keep vnet header zeroed after processing XDP

virtio-net: keep vnet header zeroed after processing XDP

[ Upstream commit 436c9453a1ac0944b82870ef2e0d9be956b396d9 ]

We copy vnet header unconditionally in page_to_skb() this is wrong
since XDP may modify the packet data. So let's keep a zeroed vnet
header for not confusing the conversion between vnet header and skb
metadata.

In the future, we should able to detect whether or not the packet was
modified and keep using the vnet header when packet was not touched.

Fixes: f600b6905015 ("virtio_net: Add XDP support")
Reported-by: Pavel Popa <pashinho1990@gmail.com>
Signed-off-by: Jason Wang <jasowang@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>