[linux 5.9.0-rc3] variable slaac v1.4

This patch can be applied to the linux kernel from this repo: git://git.kernel.org/pub/scm/linux/kernel/git/netdev/net.git
Precise release: https://git.kernel.org/pub/scm/linux/kernel/git/netdev/net.git/snapshot/net-5.9-rc3.tar.gz