ipv6 route: Prefer outgoing interface with source address assigned.

Outgoing interface is selected by the route decision if unspecified.
Let's prefer routes via interface(s) with the address assigned if we
have multiple routes with same cost.
With help from Naohiro Ooiwa <nooiwa@miraclelinux.com>.

Signed-off-by: YOSHIFUJI Hideaki <yoshfuji@linux-ipv6.org>