net/mlx5_vdpa: Offer VIRTIO_NET_F_MTU when setting MTU

Make sure to offer VIRTIO_NET_F_MTU since we configure the MTU based on
what was queried from the device.

This allows the virtio driver to allocate large enough buffers based on
the reported MTU.

Signed-off-by: Eli Cohen <elic@nvidia.com>
Link: https://lore.kernel.org/r/20211124170949.51725-1-elic@nvidia.com


Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Acked-by: Jason Wang <jasowang@redhat.com>
Reviewed-by: Si-Wei Liu <si-wei.liu@oracle.com>