routing: fix source address selection rules for IPv4 over IPv6.

Current logic always selects an IFA of the same family from the
outgoing interfaces. In IPv4 over IPv6 setup there can be just
single non-127.0.0.1 ifa, attached to the loopback interface.

Create a separate rt_getifa_family() to handle entire ifa selection
for the IPv4 over IPv6.

Differential Revision: https://reviews.freebsd.org/D31868
MFC after: 1 week