datapath: add likely in flow_lookup

datapath: add likely in flow_lookup

Upstream commit:
    commit 0a3e01371db17d753dd92ec4d0fc6247412d3b01
    Author: Tonghao Zhang <xiangxia.m.yue@gmail.com>
    Date:   Fri Nov 1 22:23:51 2019 +0800

    net: openvswitch: add likely in flow_lookup

    The most case *index < ma->max, and flow-mask is not NULL.
    We add un/likely for performance.

    Signed-off-by: Tonghao Zhang <xiangxia.m.yue@gmail.com>
    Tested-by: Greg Rose <gvrose8192@gmail.com>
    Acked-by: William Tu <u9012063@gmail.com>
    Acked-by: Pravin B Shelar <pshelar@ovn.org>
    Signed-off-by: David S. Miller <davem@davemloft.net>
Reviewed-by: Tonghao Zhang <xiangxia.m.yue@gmail.com>
Acked-by: Yi-Hung Wei <yihung.wei@gmail.com>
Signed-off-by: Greg Rose <gvrose8192@gmail.com>
Signed-off-by: Ilya Maximets <i.maximets@ovn.org>