udp: fix receiving fraglist GSO packets

[ Upstream commit b128ed5ab27330deeeaf51ea8bb69f1442a96f7f ]

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

BUG=b/365630179
TEST=presubmit
RELEASE_NOTE=None

Fixes: 89add40066f9 ("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.name
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>
Change-Id: Id1ec7ba6cd690428a551890116b7e46358076d3c
Reviewed-on: https://cos-review.googlesource.com/c/third_party/kernel/+/80582
Tested-by: Cusky Presubmit Bot <presubmit@cos-infra-prod.iam.gserviceaccount.com>
Reviewed-by: Robert Kolchmeyer <rkolchmeyer@google.com>

• net/ipv4/udp_offload.c[diff]