datapath: Fix comments for skb->_nfct

datapath: Fix comments for skb->_nfct

Upstream commit:

    commit 5e17da634a21b1200853fe82ba67d6571f2beabe
    Author: Jarno Rajahalme <jarno@ovn.org>
    Date:   Thu Feb 9 11:21:52 2017 -0800

    openvswitch: Fix comments for skb->_nfct

    Fix comments referring to skb 'nfct' and 'nfctinfo' fields now that
    they are combined into '_nfct'.

    Signed-off-by: Jarno Rajahalme <jarno@ovn.org>
    Acked-by: Pravin B Shelar <pshelar@ovn.org>
    Acked-by: Joe Stringer <joe@ovn.org>
    Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Jarno Rajahalme <jarno@ovn.org>
Acked-by: Joe Stringer <joe@ovn.org>