Update libgsystem, use it to set dirfd-relative xattrs on symlinks

This is a bit more efficient in that we're not walking full paths, and
it helps avoid security/reliability issues if an attacker (or just a
misbehaving process) has the ability to mutate paths in the middle.

src/libostree/ostree-repo-commit.c

@@ -87,12 +87,10 @@ commit_loose_object_trusted (OstreeRepo        *self,
           goto out;
         }
               
-      /* Sadly we can't use at-relative API for symlink xattrs
-       * because there's no lsetxattrat.
-       */
       if (xattrs != NULL)
         {
-          if (!gs_file_set_all_xattrs (temp_file, xattrs, cancellable, error))
+          if (!gs_dfd_and_name_set_all_xattrs (self->tmp_dir_fd, temp_filename,
+                                               xattrs, cancellable, error))
             goto out;
         }
     }