Otherwise, UDP checksum offloads could corrupt ESP packets by attempting
to calculate UDP checksum when this inner UDP packet is already protected
by IPsec.

One way to reproduce this bug is to have a VM with virtio_net driver (UFO
set to ON in the guest VM); and then encapsulate all guest's Ethernet
frames in Geneve; and then further encrypt Geneve with IPsec.  In this
case following symptoms are observed:
1. If using ixgbe NIC, then it will complain with following error message:
   ixgbe 0000:01:00.1: partial checksum but l4 proto=32!
2. Receiving IPsec stack will drop all the corrupted ESP packets and
   increase XfrmInStateProtoError counter in /proc/net/xfrm_stat.
3. iperf UDP test from the VM with packet sizes above MTU will not work at
   all.
4. iperf TCP test from the VM will get ridiculously low performance because.
Signed-off-by: Ansis Atteka <aatteka@ovn.org>
Co-authored-by: Steffen Klassert <steffen.klassert@secunet.com>
Signed-off-by: David S. Miller <davem@davemloft.net>