dm persistent data: tidy transaction manager creation fns

Tidy the transaction manager creation functions.

They no longer lock the superblock.  Superblock locking is pulled out to
the caller.

Also export dm_bm_write_lock_zero.

Signed-off-by: Joe Thornber <ejt@redhat.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>
Signed-off-by: Alasdair G Kergon <agk@redhat.com>
diff --git a/drivers/md/persistent-data/dm-block-manager.c b/drivers/md/persistent-data/dm-block-manager.c
index c70ad6e..4b5c504 100644
--- a/drivers/md/persistent-data/dm-block-manager.c
+++ b/drivers/md/persistent-data/dm-block-manager.c
@@ -565,6 +565,7 @@
 
 	return 0;
 }
+EXPORT_SYMBOL_GPL(dm_bm_write_lock_zero);
 
 int dm_bm_unlock(struct dm_block *b)
 {