net: fix setting csum_start in skb_segment()

Dave Jones reported that a crash is occurring in

csum_partial
tcp_gso_segment
inet_gso_segment
? update_dl_migration
skb_mac_gso_segment
__skb_gso_segment
dev_hard_start_xmit
sch_direct_xmit
__dev_queue_xmit
? dev_hard_start_xmit
dev_queue_xmit
ip_finish_output
? ip_output
ip_output
ip_forward_finish
ip_forward
ip_rcv_finish
ip_rcv
__netif_receive_skb_core
? __netif_receive_skb_core
? trace_hardirqs_on
__netif_receive_skb
netif_receive_skb_internal
napi_gro_complete
? napi_gro_complete
dev_gro_receive
? dev_gro_receive
napi_gro_receive

It looks like a likely culprit is that SKB_GSO_CB()->csum_start is
not set correctly when doing non-scatter gather. We are using
offset as opposed to doffset.

Reported-by: Dave Jones <davej@redhat.com>
Tested-by: Dave Jones <davej@redhat.com>
Signed-off-by: Tom Herbert <therbert@google.com>
Signed-off-by: Eric Dumazet <edumazet@google.com>
Fixes: 7e2b10c1 ("net: Support for multiple checksums with gso")
Acked-by: Tom Herbert <therbert@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>