net: ipv6: fix UDPv6 GSO segmentation with NAT

[ Upstream commit b936a9b8 ]

If any address or port is changed, update it in all packets and recalculate
checksum.

Fixes: 9fd1ff5d ("udp: Support UDP fraglist GRO/GSO.")
Signed-off-by: Felix Fietkau <nbd@nbd.name>
Reviewed-by: Willem de Bruijn <willemb@google.com>
Link: https://patch.msgid.link/20250426153210.14044-1-nbd@nbd.name


Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>