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>