NMDevice detects the DAD failures by watching the removal of tentative
addresses (happens for DAD of addresses with valid lifetime, typically
discovered addresses) or changes to addresses with dadfailed flag (permanent
addresses, typically link-local and manually configured addresses).
It retries creation of link-local addresses itself and lets RDisc know about
the rest so that it can decide if it's rdisc-managed address and retry
with a new address.

Currently NMDevice doesn't do anything useful about link-local address DAD
failures -- it just fails the link-local address addition instead of just
timing out, which happened before. RDisc just logs a warning and removes
the address from the list.

However, with RFC7217 stable privacy addresses the use of a different address
and thus a recovery from DAD failures would be possible.