datapath: __nf_ct_l{3,4}proto_find() always return a valid pointer

datapath: __nf_ct_l{3,4}proto_find() always return a valid pointer

Upstream commit:
    commit 3b78155b1b3688dbe910fecdc3e003f431b46630
    Author: Pablo Neira Ayuso <pablo@netfilter.org>
    Date:   Tue May 3 11:13:29 2016 +0200

    openvswitch: __nf_ct_l{3,4}proto_find() always return a valid pointer

    If the protocol is not natively supported, this assigns generic protocol
    tracker so we can always assume a valid pointer after these calls.

    Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
    Acked-by: Jarno Rajahalme <jrajahalme@nicira.com>
    Acked-by: Joe Stringer <joe@ovn.org>
    Signed-off-by: Jarno Rajahalme <jarno@ovn.org>
Signed-off-by: Jarno Rajahalme <jarno@ovn.org>
Acked-by: Jesse Gross <jesse@kernel.org>