bcache: Avoid deadlocking in garbage collection

Not a complete fix - we could still deadlock if btree_insert_node() has
to split...

Signed-off-by: Kent Overstreet <kmo@daterainc.com>
diff --git a/drivers/md/bcache/super.c b/drivers/md/bcache/super.c
index 7ab926d..e21200e 100644
--- a/drivers/md/bcache/super.c
+++ b/drivers/md/bcache/super.c
@@ -1601,7 +1601,7 @@
 			goto err;
 
 		err = "cannot allocate new btree root";
-		c->root = bch_btree_node_alloc(c, 0);
+		c->root = bch_btree_node_alloc(c, 0, true);
 		if (IS_ERR_OR_NULL(c->root))
 			goto err;