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