unix: handle sockets with USK_CALLBACK as external sockets

The USK_CALLBACK flag means that a socket is externel and will be
restored by a plugin. open_unixsk_standalone should not be called to
these sockets.

$ make -C test/others/unix-callback/ run
...
(00.109338)   7471: sk unix: Opening standalone socket (id 0xd ino 0 peer 0x63b)
(00.109376)   7471: Error (criu/sk-unix.c:1128): sk unix: BUG at criu/sk-unix.c:1128
Acked-by: Cyrill Gorcunov <gorcunov@openvz.org>
Signed-off-by: Andrei Vagin <avagin@virtuozzo.com>

criu/sk-unix.c

@@ -1786,7 +1786,7 @@ int add_fake_unix_queuers(void)
 	struct unix_sk_info *ui;
 
 	list_for_each_entry(ui, &unix_sockets, list) {
-		if ((ui->ue->uflags & USK_EXTERN) || ui->queuer)
+		if ((ui->ue->uflags & (USK_EXTERN | USK_CALLBACK)) || ui->queuer)
 			continue;
 		if (!(ui->ue->state == TCP_ESTABLISHED && !ui->peer) &&
 		     ui->ue->type != SOCK_DGRAM)