Modify nd6_llinfo_timer() to acquire the nd6 lock before the LLE lock.

When expiring a neighbour cache entry we may need to look up the associated
default router, which requires the nd6 read lock. To avoid an LOR, the nd6
lock should be acquired first.

X-MFC-With: r296063
Tested by: Larry Rosenman <ler@lerctr.org> (previous revision)