To16 converts the IP address ip to a 16-byte representation

https://golang.org/pkg/net/#IP.To16

Is this what you want here?

When family is != FAMILY_v4 it is v6 and v6 is 128 bits aka 16 bytes. Why shouldn't I want that?

Given type IP []byte and var srcAddr []byte
When family != FAMILY_V4, srcAddr = options.SrcAddr.To16() is as good as srcAddr = options.SrcAddr

That assumption can be wrong though when a user specifies an IPv4 source address when doing a route lookup for an IPv6 target.

Sure, in the end it would of course just not work. But at least the Netlink message is correct this way.

That assumption can be wrong though when a user specifies an IPv4 source

As far as I know a func IPv4(a, b, c, d byte) IP and func ParseIP(<an IPv4 string here") IP will generate the same IPv4-in-IPv6 mapped 16 byte slice.
You need to try a bit harder, but yes you can create a 4 byte slice net.IP, by type converting a 4 byte slice:

b := []byte{10,10,10,1}
ip := net.IP(b)

although this is not the idiomatic way to create an IP in go.
If instead the caller passes a .To4() returned net.IP, then I'd say he intentionally wants things to fail.

This net.IP handling thing which internally stores a IPv4 address into a 16 byte slice net.IP variable, but then it also gives you a To4() method which will return a 4 byte slice net.IP variable is confusing. Looks like other people think the same golang/go#18804

We just need to be careful to not use the a.To16() != nil check as we use the a.To4() != nil check: The latter will tell whether a is a IPv4 address, while the former will not always tell you that ais a IPv6 address.

Anyhow, I see this unnecessary .To16() in several other places in this library.