Not compiling in and not initializing from inetsw from in_proto.c for
IPv6 only, we need to initialize upper layer protocols from inet6sw.
Make sure to not initialize them twice in a Dual-Stack
environment but only conditionally on no INET as we have done for
TCP for a long time. Otherwise we would leak resources.
Reviewed by: gnn
Sponsored by: The FreeBSD Foundation
Sponsored by: iXsystems
MFC after: 3 days