btrfs: fix error labels in init_btrfs_fs

btrfs_interface_init rarely fails but we could leak the prelim_ref slab.

Signed-off-by: David Sterba <dsterba@suse.cz>
diff --git a/fs/btrfs/super.c b/fs/btrfs/super.c
index b1d2a42..b915d77 100644
--- a/fs/btrfs/super.c
+++ b/fs/btrfs/super.c
@@ -1999,11 +1999,11 @@
 
 	err = btrfs_prelim_ref_init();
 	if (err)
-		goto free_prelim_ref;
+		goto free_delayed_ref;
 
 	err = btrfs_interface_init();
 	if (err)
-		goto free_delayed_ref;
+		goto free_prelim_ref;
 
 	btrfs_init_lockdep();