dm: export struct dm_dev

Split struct dm_dev in two and publish the part that other targets need in
include/linux/device-mapper.h.

Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>
Signed-off-by: Alasdair G Kergon <agk@redhat.com>
diff --git a/drivers/md/dm.h b/drivers/md/dm.h
index 1e59a0b..65f2f56 100644
--- a/drivers/md/dm.h
+++ b/drivers/md/dm.h
@@ -25,13 +25,10 @@
 /*
  * List of devices that a metadevice uses and should open/close.
  */
-struct dm_dev {
+struct dm_dev_internal {
 	struct list_head list;
-
 	atomic_t count;
-	int mode;
-	struct block_device *bdev;
-	char name[16];
+	struct dm_dev dm_dev;
 };
 
 struct dm_table;