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>

drivers/md/dm-thin-metadata.c

@@ -614,7 +614,7 @@ static int __commit_transaction(struct dm_pool_metadata *pmd)
 	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;