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 <[e-mail]>
Acked-by: Willem de Bruijn <[e-mail]>
Signed-off-by: David S. Miller <[e-mail]>

net/core/dev.c

···
 	/* To get more precise estimation of bytes sent on wire,
 	 * we add to pkt_len the headers size of all segments
 	 */
-	if (shinfo->gso_size)  {
+	if (shinfo->gso_size && skb_transport_header_was_set(skb)) {
 		unsigned int hdr_len;
 		u16 gso_segs = shinfo->gso_segs;