Add refcounting to IPv6 DAD objects and simplify the DAD code to fix a
number of races which could cause double frees or use-after-frees when
performing DAD on an address. In particular, an IPv6 address can now only be
marked as a duplicate from the DAD callout.

Differential Revision: https://reviews.freebsd.org/D1258
Reviewed by: ae, hrs
Reported by: rstone
MFC after: 1 month