system-userspace-macros: Fix ethtool with new kernels.

system-userspace-macros: Fix ethtool with new kernels.

The latest net-next kernels have removed the UFO feature, which results
in older ethtool reporting the following error:

Cannot get device udp-fragmentation-offload settings: Operation not
supported

Currently, we rely on no errors being reported, and if there is an error
then a failure is reported. However, in this case we can safely ignore
the stderr output. We still check the return code so if something is
truly fatal, a failure will still be reported; otherwise, we will not
fail the test due to the above.

Signed-off-by: Joe Stringer <joe@ovn.org>
Acked-by: Andy Zhou <azhou@ovn.org>