This change adds a flag to the DAD entry to indicate whether it is
currently on the queue. This prevents accidentally doubly-removing a DAD
entry from the queue, while also simplifying some of the logic in
nd6_dad_stop().

Reviewed by: ae, hrs, vangyzen
MFC after: 2 weeks
Sponsored by: Netflix, Inc.
Differential Revision: https://reviews.freebsd.org/D10943