dm cache: move hook_info into common portion of per_bio_data structure

Commit c9d28d5d ("dm cache: promotion optimisation for writes")
incorrectly placed the 'hook_info' member in the writethrough-only
portion of the per_bio_data structure.

Given that the overwrite optimization may be used for writeback the
'hook_info' member must be placed above the 'cache' member of the
per_bio_data structure.  Any members above 'cache' are available from
both writeback and writethrough modes' per_bio_data structure.

Signed-off-by: Mike Snitzer <snitzer@redhat.com>
Acked-by: Joe Thornber <ejt@redhat.com>
Cc: stable@vger.kernel.org # 3.13+

drivers/md/dm-cache-target.c

@@ -289,6 +289,7 @@ struct per_bio_data {
 	bool tick:1;
 	unsigned req_nr:2;
 	struct dm_deferred_entry *all_io_entry;
+	struct dm_hook_info hook_info;
 
 	/*
 	 * writethrough fields.  These MUST remain at the end of this
@@ -297,7 +298,6 @@ struct per_bio_data {
 	 */
 	struct cache *cache;
 	dm_cblock_t cblock;
-	struct dm_hook_info hook_info;
 	struct dm_bio_details bio_details;
 };