Lock the ND prefix list and add refcounting for prefixes.

This change extends the nd6 lock to protect the ND prefix list as well
as the list of advertising routers associated with each prefix. To handle
cases where the nd6 lock must be dropped while iterating over either the
prefix or default router lists, a generation counter is used to track
modifications to the lists. Additionally, a new mutex is used to serialize
prefix on-link/off-link transitions. This mutex must be acquired before
the nd6 lock and is held while updating the routing table in
nd6_prefix_onlink() and nd6_prefix_offlink().

Reviewed by: ae, tuexen (SCTP bits)
Tested by: Jason Wolfe <jason@llnw.com>,

		Larry Rosenman <ler@lerctr.org>

MFC after: 2 months
Differential Revision: https://reviews.freebsd.org/D8125