kernel - Fix namecache memory exhaustion edge case

kernel - Fix namecache memory exhaustion edge case

* The namecache has traditionally relied on vnode limits to avoid
  blowing up its memory pool.  However, creating millions of hardlinks
  to the same vnode easily circumvents this assumption.

* Enhance the negative cache hysteresis code with a section for the
  positive cache that does the same thing, preventing the namecache
  from growing greater than 2x maxvnodes.

  This situation will not occur under normal operation, it is strictly
  for correctness.  The kernel should never panic on normal filesystem
  operations even if those operations are unrealistic.

* Tested w/HAMMER and a few million hardlinks to a single file.

Reported-by: Venkatesh Srinivas <me@acm.jhu.edu>