Try to ensure kernel IPv6 link local address generation occurs by
setting the per-if addr_gen_mode sysctl when the link is already up,
instead of the netlink interface (IFLA_INET6_ADDR_GEN_MODE).
The netlink setting is sufficient in cases where the interface is not
yet up when networkd configures an interface - bringing the interface
up will trigger in-kernel address generation.
If the interface is already up, yet the interface has no IPv6LL assigned
setting IFLA_INET6_ADDR_GEN_MODE has no effect.
Writing the addr_gen_mode sysctl is a best effort attempt at triggering
address generation regardless of interface state because it also works
in cases where the interface is already up.
Fixes#22424.
