Add a check, that is currently under discussion for 8 but that we need
to keep for 7-STABLE when MFCing in_pcbladdr() to not change the
behaviour there.

With this a destination route via a loopback interface is treated as
a valid and reachable thing for IPv4 source address selection, even
though nothing of that network is ever directly reachable, but it is
more like a blackhole route.
With this the source address will be selected and IPsec can grab the
packets before we would discard them at a later point, encapsulate them
and send them out from a different tunnel endpoint IP.

Discussed on: net
Reported by: Frank Behrens <frank@harz.behrens.de>
Tested by: Frank Behrens <frank@harz.behrens.de>
MFC after: 4 weeks (just so that I get the mail)