Lock the NDP default router list and count defrouter references.

This addresses a number of race conditions that can cause crashes as a
result of unsynchronized access to the list.

PR: 206904
Tested by: Larry Rosenman <ler@lerctr.org>,

		Kevin Bowling <kevin.bowling@kev009.com>

MFC after: 2 months
Differential Revision: https://reviews.freebsd.org/D5315