udp: fix receiving fraglist GSO packets

commit b128ed5ab27330deeeaf51ea8bb69f1442a96f7f upstream.

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: 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: Greg Kroah-Hartman <gregkh@linuxfoundation.org>