datapath: Add support for RHEL-7 / CentOS-7 kernel.

This patch mostly is related to tunnel API where RHEL 7
kernel API are not in-sync with newer linux kernel API. So
extra checks are required to check for parameters of API.

Signed-off-by: Pravin B Shelar <pshelar@nicira.com>
Acked-by: Jiri Benc <jbenc@redhat.com>