cgroups: check for correct error in __cg_unified_attach() from cgroup_attach()

src/lxc/cgroups/cgfsng.c

@@ -2415,7 +2415,7 @@ static int __cg_unified_attach(const struct hierarchy *h,
 	ret = cgroup_attach(conf, name, lxcpath, pid);
 	if (ret == 0)
 		return log_trace(0, "Attached to unified cgroup via command handler");
-	if (ret != -EBADF)
+	if (ret != -ENOCGROUP2)
 		return log_error_errno(ret, errno, "Failed to attach to unified cgroup");
 
 	/* Fall back to retrieving the path for the unified cgroup. */