apparmor: check label build before no_new_privs test

In the Linux kernel, the following vulnerability has been resolved: apparmor: check label build before no_new_privs test aa_change_profile() builds a replacement label with fn_label_build_in_scope() before the no_new_privs subset check. The build helper can fail and return NULL or an ERR_PTR, but the result was passed to aa_label_is_unconfined_subset() before the existing IS_ERR_OR_NULL() check. Reuse the existing target-label build failure handling immediately after the build. This preserves the current audit handling while preventing the subset helper from dereferencing an invalid label.