netlink: Handle RTA_PREFSRC on RTM_NEWROUTE

[email_luiz.eng.br]

Summary

While investigating a problem with the source address selection for IPv4 over IPv6 nexthop routes, I decided to try setting the prefsrc on the routes exported from BIRD, only to realize that there was no support for it.

With this patch, I was able to set a proper source address for routes added via Netlink.

Sponsored by: InnoGames GmbH

[melifaro]

Thank you for working on this!
Could you please add the patch with context & add a corresponding test?
Thank you!

[email_luiz.eng.br]

In D46796#1067574, @melifaro wrote:

Could you please add the patch with context & add a corresponding test?

Will do, just give me a few days for the tests.