datapath: Use kernel Geneve implementation on 4.0 and above.

datapath: Use kernel Geneve implementation on 4.0 and above.

When Geneve was originally backported, it wasn't available as part
of a released kernel version but it is now, so we can take advantage
of the native implementation.

Note that Geneve was actually first available as part of the 3.18
kernel release but some drivers erroreously try to offload it as
if it were VXLAN, which was fixed in the 4.0 release. Since our
UDP tunnel compat layer already takes care of this, we continue
using the OVS Geneve implementation until 4.0.

Reported-by: Alex Wang <alexw@nicira.com>
Signed-off-by: Jesse Gross <jesse@nicira.com>
Reviewed-by: Simon Horman <simon.horman@netronome.com>