datapath: Enable OVS GSO to be used up to 3.18 if necessary.

datapath: Enable OVS GSO to be used up to 3.18 if necessary.

There are two important GSO tunnel features that were introduced
after the 3.12 cutoff for our current out of tree GSO implementation:
 * 3.16 introduced support for outer UDP checksums.
 * 3.18 introduced support for verifying hardware support for protocols
   other than VXLAN.

In cases where these features are used, we should use OVS GSO to
ensure correct behavior. However, we also want to continue to use
kernel GSO or hardware TSO in existing situations. Therefore, this
extends the range of kernels where OVS GSO is available to 3.18 and
makes it easier to select which one to use.

Signed-off-by: Jesse Gross <jesse@nicira.com>
Acked-by: Thomas Graf <tgraf@noironetworks.com>
Acked-by: Pravin B Shelar <pshelar@nicira.com>