MFC r296063 r297397 r299213

296063:
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.

297397
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.

299213
Clean up callers of nd6_prelist_add().

nd6_prelist_add() sets *newp if and only if it is successful, so there's no
need for code that handles the case where the return value is 0 and
*newp == NULL. Fix some style bugs in nd6_prelist_add() while here.

Submitted by: Jason Wolfe <j@nitrology.com>