datapath: Interface with NAT.

datapath: Interface with NAT.

Upstream commit:
    commit 05752523e56502cd9975aec0a2ded465d51a71f3
    Author: Jarno Rajahalme <jarno@ovn.org>
    Date:   Thu Mar 10 10:54:23 2016 -0800

    openvswitch: Interface with NAT.

    Extend OVS conntrack interface to cover NAT.  New nested
    OVS_CT_ATTR_NAT attribute may be used to include NAT with a CT action.
    A bare OVS_CT_ATTR_NAT only mangles existing and expected connections.
    If OVS_NAT_ATTR_SRC or OVS_NAT_ATTR_DST is included within the nested
    attributes, new (non-committed/non-confirmed) connections are mangled
    according to the rest of the nested attributes.

    The corresponding OVS userspace patch series includes test cases (in
    tests/system-traffic.at) that also serve as example uses.

    This work extends on a branch by Thomas Graf at
    https://github.com/tgraf/ovs/tree/nat.

    Signed-off-by: Jarno Rajahalme <jarno@ovn.org>
    Acked-by: Thomas Graf <tgraf@suug.ch>
    Acked-by: Joe Stringer <joe@ovn.org>
    Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
Signed-off-by: Jarno Rajahalme <jarno@ovn.org>
Acked-by: Jesse Gross <jesse@kernel.org>