dm: add presuspend_undo hook to target_type

The DM thin-pool target now must undo the changes performed during
pool_presuspend() so introduce presuspend_undo hook in target_type.

Signed-off-by: Mike Snitzer <snitzer@redhat.com>
Acked-by: Joe Thornber <ejt@redhat.com>
diff --git a/include/linux/device-mapper.h b/include/linux/device-mapper.h
index e1707de..ca6d2ac 100644
--- a/include/linux/device-mapper.h
+++ b/include/linux/device-mapper.h
@@ -64,6 +64,7 @@
 				    union map_info *map_context);
 
 typedef void (*dm_presuspend_fn) (struct dm_target *ti);
+typedef void (*dm_presuspend_undo_fn) (struct dm_target *ti);
 typedef void (*dm_postsuspend_fn) (struct dm_target *ti);
 typedef int (*dm_preresume_fn) (struct dm_target *ti);
 typedef void (*dm_resume_fn) (struct dm_target *ti);
@@ -145,6 +146,7 @@
 	dm_endio_fn end_io;
 	dm_request_endio_fn rq_end_io;
 	dm_presuspend_fn presuspend;
+	dm_presuspend_undo_fn presuspend_undo;
 	dm_postsuspend_fn postsuspend;
 	dm_preresume_fn preresume;
 	dm_resume_fn resume;