kernel - Fix filesystem lookup error due to parent directory recyclement race

kernel - Fix filesystem lookup error due to parent directory recyclement race

* When looking up a path the parent ncp's vnode is needed to pass into
  the VFS code as the directory vnode (dvp) for the element being looked up.

* Fix a timing race whereby a system under extreme vnode pressure (such as
  when kern.maxvnodes is set to a very low value) can squeek in recyclement
  of this directory vnode when there are no children under it in the
  namecache.

  We fix the problem by holding the directory vnode during the nlookup() and
  cache_resolve().