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.