dm thin: defer whole cells rather than individual bios

This avoids dropping the cell, so increases the probability that other
bios will collect within the cell, rather than being passed individually
to the worker.

Also add required process_cell and process_discard_cell error handling
wrappers and set associated pool-mode function pointers accordingly.
Signed-off-by: Joe Thornber <ejt@redhat.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>

drivers/md/dm-bio-prison.h

@@ -35,6 +35,7 @@ struct dm_cell_key {
  * themselves.
  */
 struct dm_bio_prison_cell {
+	struct list_head user_list;	/* for client use */
 	struct rb_node node;
 
 	struct dm_cell_key key;