ip6_output: use new routing KPI when not passed a cached route

Implement the equivalent of r347375 (IPv4) for the IPv6 output path.
In IPv6 we get passed a cached route (and inp) by udp6_output()
depending on whether we acquired a write lock on the INP.
In case we neither bind nor connect a first UDP packet would come in
with a cached route (wlocked) and all further packets would not.
In case we bind and do not connect we never write-lock the inp.

When we do not pass in a cached route, rather than providing the
storage for a route locally and pass it over the old lookup code
and down the stack, use the new route lookup KPI and acquire all
details we need to send the packet.

Compared to the IPv4 code the IPv6 code has a couple of possible
complications: given an option with a routing hdr/caching route there,
and path mtu (ro_pmtu) case which now equally has to deal with the
possibility of having a route which is NULL passed in, and the
fwd_tag in case a firewall changes the next hop (something to
factor out in the future).

Sponsored by: Netflix
Reviewed by: glebius
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D23886