datapath: Find existing conntrack entry after upcall.

datapath: Find existing conntrack entry after upcall.

Upstream commit:
    commit 289f225349cb2a97448fd14599ab34b741f706f3
    Author: Jarno Rajahalme <jarno@ovn.org>
    Date:   Thu Mar 10 10:54:20 2016 -0800

    openvswitch: Find existing conntrack entry after upcall.

    Add a new function ovs_ct_find_existing() to find an existing
    conntrack entry for which this packet was already applied to.  This is
    only to be called when there is evidence that the packet was already
    tracked and committed, but we lost the ct reference due to an
    userspace upcall.

    ovs_ct_find_existing() is called from skb_nfct_cached(), which can now
    hide the fact that the ct reference may have been lost due to an
    upcall.  This allows ovs_ct_commit() to be simplified.

    This patch is needed by later "openvswitch: Interface with NAT" patch,
    as we need to be able to pass the packet through NAT using the
    original ct reference also after the reference is lost after an
    upcall.

    Signed-off-by: Jarno Rajahalme <jarno@ovn.org>
    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>