virtio-net: enable NETIF_F_GRO_HW only if GRO-related offloads are supported

virtio-net: enable NETIF_F_GRO_HW only if GRO-related offloads are supported

Negotiating VIRTIO_NET_F_CTRL_GUEST_OFFLOADS indicates the device
allows control over offload support, but the offloads that can be
controlled may have nothing to do with GRO (e.g., if neither GUEST_TSO4
nor GUEST_TSO6 is supported).

In such a setup, reporting NETIF_F_GRO_HW as available for the device
is too optimistic and misleading to the user.

Improve the situation by masking off NETIF_F_GRO_HW unless the device
possesses actual GRO-related offload capabilities. Out of an abundance
of caution, this does not change the current behaviour for hardware with
just v6 or just v4 GRO: current interfaces do not allow distinguishing
between v6/v4 GRO, so we can't expose them to userspace precisely.

Signed-off-by: Di Zhu <zhud@hygon.cn>
Acked-by: Jason Wang <jasowang@redhat.com>
Link: https://patch.msgid.link/20260323041730.986351-1-zhud@hygon.cn
Signed-off-by: Jakub Kicinski <kuba@kernel.org>