Stop using rt_gwroute

[Ryota Ozaki <ozaki-r%netbsd.org@localhost>]

Hi,

rt_gwroute of rtentry is a reference to a rtentry of the gateway
for a rtentry with RTF_GATEWAY. That was used by L2 (arp and ndp)
to look up L2 addresses. By separating L2 nexthop caches, we don't
need a route for the purpose and we can stop using rt_gwroute.
By doing so, we can reduce referencing and modifying rtentries,
which makes it easy to applying a lock (and/or psref) to the
routing table and rtentries.

Here is a patch:
  http://www.netbsd.org/~ozaki-r/no-rt_gwroute.diff

One issue to do this is to keep RTF_REJECT behavior. It seems it
was broken when we moved rtalloc1 things from L2 output routines
(e.g., ether_output) to ip_hresolv_output, but (fortunately?)
it works unexpectedly. What we mistook are:
- RTF_REJECT was checked for any routes in L2 output routines,
  but in ip_hresolv_output it is checked only when the route
  is RTF_GATEWAY
- The RTF_REJECT check wasn't moved to IPv6 (nd6_output)

It seems that rt_gwroute checks hid the mistakes and it looked
work (unexpectedly) and removing rt_gwroute checks unveil the
issue. So we need to fix RTF_REJECT checks in ip_hresolv_output
and also add them to nd6_output.

One more point we have to care is returning an errno; we need
to mimic looutput behavior. Originally RTF_REJECT check was
done either in L2 output routines or in looutput. The latter is
applied when a reject route directs to a loopback interface.
However, now RTF_REJECT check is done before looutput so to keep
the original behavior we need to return an errno which looutput
chooses. Added rt_check_reject_route does such tweaks.

Any comments or suggestions?

Thanks,
  ozaki-r