openvswitch: Ensure flow is valid before executing ct

The ct action uses parts of the flow key, so we need to ensure that it
is valid before executing that action.

Fixes: 7f8a436e "openvswitch: Add conntrack action"
Signed-off-by: Joe Stringer <joestringer@nicira.com>
Acked-by: Pravin B Shelar <pshelar@nicira.com>
Signed-off-by: David S. Miller <davem@davemloft.net>