union checkout: Fix symlink handling for xattrs

Applying xattrs on a symlink during checkout failed since
it was setting the xattrs on the final filename, not the
temporary name.

This made the "checkout union 1" test in test-basic.sh
fail.

https://bugzilla.gnome.org/show_bug.cgi?id=741125

src/libostree/ostree-repo-checkout.c

@@ -271,7 +271,7 @@ checkout_file_unioning_from_input_at (OstreeRepoCheckoutMode mode,
           
       if (xattrs)
         {
-          if (!gs_dfd_and_name_set_all_xattrs (destination_dfd, destination_name,
+          if (!gs_dfd_and_name_set_all_xattrs (destination_dfd, temp_filename,
                                                xattrs, cancellable, error))
             goto out;
         }