virtio_net: do not allow tunnel csum offload for non GSO packets

virtio_net: do not allow tunnel csum offload for non GSO packets

Fiona reports broken connectivity for virtio net setup using UDP tunnel
inside the guest and NIC with not UDP tunnel TSO support in the host.

Currently the virtio_net driver exposes csum offload for UDP-tunneled,
TCP non GSO packets. Such packet reach the host as CSUM_PARTIAL ones
with the 'encapsulation' flag cleared, as the virtio specification do
not support this specific kind of offload.

HW NICs with UDP tunnel TSO support - and those drivers directly
accessing skb->csum_start/csum_offset - are still capable of computing
the needed csum correctly, but otherwise the packets reach the wire with
bad csum on both the inner and outer transport header.

Address the issue explicitly disabling csum offload for UDP tunneled,
non GSO packets via the ndo_features_check op.

Fixes: 56a06bd40fab ("virtio_net: enable gso over UDP tunnel support.")
Reported-by: Fiona Ebner <f.ebner@proxmox.com>
Closes: https://bugzilla.proxmox.com/show_bug.cgi?id=7627
Tested-by: Fiona Ebner <f.ebner@proxmox.com>
Tested-by: Gabriel Goller <g.goller@proxmox.com>
Acked-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Reviewed-by: Gabriel Goller <g.goller@proxmox.com>
Tested-by: Gabriel Goller <g.goller@proxmox.com>
Link: https://patch.msgid.link/6c3b6c47fb05c100f384630dc48f3975cf37b67a.1781195144.git.pabeni@redhat.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>