openvswitch: avoid resetting flow key while installing new flow.

since commit commit db74a333

 ("openvswitch: use percpu
flow stats") flow alloc resets flow-key. So there is no need
to reset the flow-key again if OVS is using newly allocated
flow-key.

Signed-off-by: Pravin B Shelar <pshelar@ovn.org>
Signed-off-by: David S. Miller <davem@davemloft.net>