datapath: add and use nf_ct_set helper

datapath: add and use nf_ct_set helper

Upstream commit:

    commit c74454fadd5ea6fc866ffe2c417a0dba56b2bf1c
    Author: Florian Westphal <fw@strlen.de>
    Date:   Mon Jan 23 18:21:57 2017 +0100

    netfilter: add and use nf_ct_set helper

    Add a helper to assign a nf_conn entry and the ctinfo bits to an sk_buff.
    This avoids changing code in followup patch that merges skb->nfct and
    skb->nfctinfo into skb->_nfct.

    Signed-off-by: Florian Westphal <fw@strlen.de>
    Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
Signed-off-by: Jarno Rajahalme <jarno@ovn.org>
Acked-by: Joe Stringer <joe@ovn.org>