dm thin metadata: pass correct space map to dm_sm_root_size

Fix a harmless typo.

The root is a chunk of data that gets written to the superblock.  This
data is used to recreate the space map when opening a metadata area.
We have two space maps; one tracking space on the metadata device and
one of the data device.  Both of these use the same format for their
root, so this typo was harmless.

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/dm-thin-metadata.c b/drivers/md/dm-thin-metadata.c
index 237571a..a680c76 100644
--- a/drivers/md/dm-thin-metadata.c
+++ b/drivers/md/dm-thin-metadata.c
@@ -614,7 +614,7 @@
 	if (r < 0)
 		goto out;
 
-	r = dm_sm_root_size(pmd->metadata_sm, &data_len);
+	r = dm_sm_root_size(pmd->data_sm, &data_len);
 	if (r < 0)
 		goto out;