datapath: Fix zero key tunnels.

datapath: Fix zero key tunnels.

Datapath tunneling check for flag OVS_FLOW_TNL_F_KEY is failing,
causing it to drop packet. This only happens on tunnels with
zero key as vswitchd does not generate set-tunnel action. Set
tunnel action sets this flags for given action. To fix this issue
the check is dropped.

Bug #13666

Signed-off-by: Pravin B Shelar <pshelar@nicira.com>
Acked-by: Kyle Mestery <kmestery@cisco.com>
Acked-by: Jesse Gross <jesse@nicira.com>