"dup" isn't a good label name, we use it for any case where the callout
won't be rescheduled.

Also fix a leak that would occur if ifa is null.
nd6_dad_init() needs to actually be called...
Merge branch 'master' into 20141118-nd6-dad-refcount
Merge branch 'markj-kernel-config' into 20141118-nd6-dad-refcount
Add a prototype.
Merge branch 'master' into markj-kernel-config
Merge branch 'markj-kernel-config' into 20141118-nd6-dad-refcount
Don't give the callout a reference - it isn't necessary.
Merge branch '20141118-nd6-dad-refcount' into markj-develop