openvswitch: Optimize flow key match for non tunnel flows.

Following patch adds start offset for sw_flow-key, so that we can
skip tunneling information in key for non-tunnel flows.

Signed-off-by: Pravin B Shelar <pshelar@nicira.com>
Acked-by: Jesse Gross <jesse@nicira.com>
Signed-off-by: David S. Miller <davem@davemloft.net>