- It is impossible to run request_cleanup() while doing a copyonwrite.

 - It is impossible to run request_cleanup() while doing a copyonwrite.
   This will most likely cause new block allocations which can recurse
   into request cleanup.
 - While here optimize the ufs locking slightly.  We need only acquire and
   drop once.
 - process_removes() and process_truncates() also is only needed once.
 - Attempt to flush each item on the worklist once but do not loop forever
   if some can not be completed.

Discussed with:	mckusick