netlink: improve source ifa selection algorithm when adding routes.

Use route destination sockaddr when the gateway is eiter AF_LINK or
has the different family (IPv4 over IPv6). This change ensures
the nexthop IFA has the same family as the destination.

Reported by: Dmitriy Smirnov <fox@sage.su>
Tested by: Dmitriy Smirnov <fox@sage.su>
MFC after: 3 days