system-tests: Disable offloads in userspace tests.

system-tests: Disable offloads in userspace tests.

The system userspace testsuite uses the userspace datapath with
netdev-linux devices, connected to veth pairs with the AF_PACKET socket:

             (veth pair)     (AF_PACKET)
TCP stack -> p0 ---> ovs-p0  -------------> netdev-linux (userspace OVS)

Unfortunately this configuration has some problems with offloads: a
packet generated by the TCP stack maybe sent to p0 without being
checksummed or segmented. The AF_PACKET socket, by default, ignores the
offloads and just transmits the data of the packets to userspace, but:

1. The packet may need GSO, so the data will be too big to be received
   by the userspace datapath
2. The packet might have incomplete checksums, so it will likely be
   discarded by the receiver.

Problem 1 causes TCP connections to see a congestion window smaller than
the MTU, which hurts performance but doesn't prevent communication.

Problem 2 was hidden in the testsuite by a Linux kernel bug, fixed by
commit ce8c839b74e3("veth: don’t modify ip_summed; doing so treats
packets with bad checksums as good").  In the kernels that include the
fix, the userspace datapath is able to process pings, but not tcp or udp
data.

Unfortunately I couldn't find a way to ask the AF_PACKET to perform
offloads in kernel.  A possible fix would be to use the PACKET_VNET_HDR
sockopt and perform the offloads in userspace.

Until a proper fix is worked out for netdev-linux, this commit disables
offloads on the non-OVS side of the veth pair, as a workaround.

Signed-off-by: Daniele Di Proietto <diproiettod@vmware.com>
Acked-by: Joe Stringer <joe@ovn.org>
Acked-by: Flavio Leitner <fbl@sysclose.org>