udp: fix receiving fraglist GSO packets

When assembling fraglist GSO packets, udp4_gro_complete does not set
skb->csum_start, which makes the extra validation in __udp_gso_segment fail.

Fixes: 89add400 ("net: drop bad gso csum_start and offset in virtio_net_hdr")
Signed-off-by: Felix Fietkau <nbd@nbd.name>
Reviewed-by: Willem de Bruijn <willemb@google.com>
Link: https://patch.msgid.link/20240819150621.59833-1-nbd@nbd.nameSigned-off-by: Jakub Kicinski <kuba@kernel.org>

net/ipv4/udp_offload.c

@@ -279,7 +279,8 @@ struct sk_buff *__udp_gso_segment(struct sk_buff *gso_skb,
 		return ERR_PTR(-EINVAL);
 
 	if (unlikely(skb_checksum_start(gso_skb) !=
-		     skb_transport_header(gso_skb)))
+		     skb_transport_header(gso_skb) &&
+		     !(skb_shinfo(gso_skb)->gso_type & SKB_GSO_FRAGLIST)))
 		return ERR_PTR(-EINVAL);
 
 	/* We don't know if egress device can segment and checksum the packet