datapath: Handle NF_NAT_NEEDED replacement

datapath: Handle NF_NAT_NEEDED replacement

Starting from the following upstream commit, NF_NAT_NEEDED is replaced
by IS_ENABLED(CONFIG_NF_NAT) in the upstream kernel. This patch makes
some changes so that our in tree ovs kernel module is compatible to
both old and new kernels.

Upstream commit:
commit 4806e975729f99c7908d1688a143f1e16d464e6c
Author: Florian Westphal <fw@strlen.de>
Date:   Wed Mar 27 09:22:26 2019 +0100

    netfilter: replace NF_NAT_NEEDED with IS_ENABLED(CONFIG_NF_NAT)

    NF_NAT_NEEDED is true whenever nat support for either ipv4 or ipv6 is
    enabled.  Now that the af-specific nat configuration switches have been
    removed, IS_ENABLED(CONFIG_NF_NAT) has the same effect.

    Signed-off-by: Florian Westphal <fw@strlen.de>
    Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
Signed-off-by: Yi-Hung Wei <yihung.wei@gmail.com>
Reviewed-by: Yifeng Sun <pkusunyifeng@gmail.com>
Signed-off-by: Ben Pfaff <blp@ovn.org>