kernel - Fix cluster_awrite() race

kernel - Fix cluster_awrite() race

* Fix a race between cluster_awrite() and vnode destruction.  We
  have to finish working the cluster pbuf before disposing of the
  component elements.  b_vp in the cluster pbuf is held only by
  the presence of the components.

* Fixes NULL pointer indirection panic associated with heavy
  paging during tmpfs operations.  This typically only occurs
  when maxvnodes is set to a relatively low value, but it can
  eventually occur in any modestly paging environment when
  tmpfs is used.