Merge common code into new nd6_dad_destroy()

Merge common code into new nd6_dad_destroy()

The current code wrt. stopping DAD for and removing a particular IP from
the list is flawed.

Introduce a single nd6_dad_destroy() to the cleanup, so that there's
only one place to fix.

This is just a mechanical deduplication without significant behaviour
change;  in case a duplicated address was found, RTM_CHGADDRATTR now goes
out before cleanup, which should be no problem.

The nd6_dad_create() pendant could be done as well, but the end of
nd6_dad_start() is currently the only place where a new IP/DAD entry is
set up, so little gain besides function name symmetry.

OK claudio