net: Skip GSO length estimation if transport header is not set

qdisc_pkt_len_init expects transport_header to be set for GSO packets.
Patch [1] skips transport_header validation for GSO packets that don't
have network_header set at the moment of calling virtio_net_hdr_to_skb,
and allows them to pass into the stack. After patch [2] no placeholder
value is assigned to transport_header if dissection fails, so this patch
adds a check to the place where the value of transport_header is used.

[1] https://patchwork.ozlabs.org/patch/1044429/
[2] https://patchwork.ozlabs.org/patch/1046122/



Signed-off-by: Maxim Mikityanskiy <maximmi@mellanox.com>
Acked-by: Willem de Bruijn <willemb@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>