Cancel DAD for an ifa when the ifp has ND6_IFF_IFDISABLED as early as
possible and do not clear IN6_IFF_TENTATIVE.  If IFDISABLED was accidentally
set after a DAD started, TENTATIVE could be cleared because no NA was
received due to IFDISABLED, and as a result it could prevent DAD when
manually clearing IFDISABLED after that.