checklocks: enforce IPv6 state ownership

IPv6 endpoint, DAD, and protocol state lack checked mutex ownership.
Annotate the fields and NDP/MLD helper contracts, adapting #7060 to the
current named state and embedded mutexes.

Alias the endpoint mutex paths through the initialized NDP and MLD
backpointers. Record narrow checklocks exceptions where synchronous
iteration preserves the caller's lock or DAD and job callbacks run
under the supplied endpoint lock.

Require a read lock for address acquisition, matching its callers and
the addressable state implementation that owns exclusive creation.

Assisted-by: Codex