SLOW_WORK: Make slow_work_ops ->get_ref/->put_ref optional

Make the ability for the slow-work facility to take references on a work item
optional as not everyone requires this.

Even the internal slow-work stubs them out, so those can be got rid of too.

Signed-off-by: Jens Axboe <jens.axboe@oracle.com>
Signed-off-by: David Howells <dhowells@redhat.com>

Documentation/slow-work.txt

@@ -125,7 +125,7 @@ ITEM OPERATIONS
 ===============
 
 Each work item requires a table of operations of type struct slow_work_ops.
-All members are required:
+Only ->execute() is required, getting and putting of a reference are optional.
 
  (*) Get a reference on an item: