bcache: try to set b->parent properly

bcache_flash_dev.ktest would reliably crash with 8k and 16k bucket size
before; now it passes.

Change-Id: Ib542232235e39298c3a7548fe52b645cabb823d1
diff --git a/drivers/md/bcache/super.c b/drivers/md/bcache/super.c
index 12ad381..b6114d6 100644
--- a/drivers/md/bcache/super.c
+++ b/drivers/md/bcache/super.c
@@ -1601,7 +1601,7 @@
 			goto err;
 
 		err = "error reading btree root";
-		c->root = bch_btree_node_get(c, NULL, k, j->btree_level, true);
+		c->root = bch_btree_node_get(c, NULL, k, j->btree_level, true, NULL);
 		if (IS_ERR_OR_NULL(c->root))
 			goto err;
 
@@ -1676,7 +1676,7 @@
 			goto err;
 
 		err = "cannot allocate new btree root";
-		c->root = __bch_btree_node_alloc(c, NULL, 0, true);
+		c->root = __bch_btree_node_alloc(c, NULL, 0, true, NULL);
 		if (IS_ERR_OR_NULL(c->root))
 			goto err;