Constify rtentry of if_output

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

Hi,

After stopping using rt_gwroute, nobody doesn't need
to change rtentry below if_output, so I did:
  http://www.netbsd.org/~ozaki-r/constify-if_output.diff

The change makes clear where rtentries are changed (or
not) and helps forthcoming locking (os psrefing) rtentries.

Let me know if I miss something.

Thanks,
  ozaki-r