linklocal6_complete() had only one caller. The caller would check
whether the conditions for linklocal6_complete() are satisfied, and
then call it. Note that linklocal6_complete() would again assert
that these conditions hold. Don't do this. Just move the check
inside linklocal6_complete(), and rename to linklocal6_check_complete().

Also, linklocal6_complete() was called by update_ip_config(),
which was called by nm_device_capture_initial_config() and
queued_ip6_config_change().
It doesn't make sense to call linklocal6_complete() during
nm_device_capture_initial_config(). Move the call to
queued_ip6_config_change().