selftests: drv-net: tso: fix vxlan tunnel flags to get correct gso_type

selftests: drv-net: tso: fix vxlan tunnel flags to get correct gso_type

When vxlan is used with ipv6 as the outer network header, the correct
ip link parameters for acheiving the SKB_GSO_UDP_TUNNEL gso type is
"udp6zerocsumtx udp6zerocsumrx". Otherwise the gso type will be
SKB_GSO_UDP_TUNNEL_CSUM.

This bug was the reason for the second of the three possible
invocations of run_one_stream() invocations, so that can be deleted as
well. We only need to test with the feature off and on.

Fixes: 0d0f4174f6c8 ("selftests: drv-net: add a simple TSO test")
Signed-off-by: Daniel Zahka <daniel.zahka@gmail.com>
Link: https://patch.msgid.link/20250723184740.4075410-3-daniel.zahka@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>