dm persistent data: eliminate unnecessary return values

dm_bm_unlock and dm_tm_unlock return an integer value but the returned
value is always 0.  The calling code sometimes checks the return value
and sometimes doesn't.

Eliminate these unnecessary return values and also the checks for them.

Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>
diff --git a/drivers/md/persistent-data/dm-block-manager.c b/drivers/md/persistent-data/dm-block-manager.c
index 88dbe7b..f2393ba 100644
--- a/drivers/md/persistent-data/dm-block-manager.c
+++ b/drivers/md/persistent-data/dm-block-manager.c
@@ -578,7 +578,7 @@
 }
 EXPORT_SYMBOL_GPL(dm_bm_write_lock_zero);
 
-int dm_bm_unlock(struct dm_block *b)
+void dm_bm_unlock(struct dm_block *b)
 {
 	struct buffer_aux *aux;
 	aux = dm_bufio_get_aux_data(to_buffer(b));
@@ -590,8 +590,6 @@
 		bl_up_read(&aux->lock);
 
 	dm_bufio_release(to_buffer(b));
-
-	return 0;
 }
 EXPORT_SYMBOL_GPL(dm_bm_unlock);