ppp: Use skb_queue_walk() in ppp_mp_insert().

	/* N.B. we don't need to lock the list lock because we have the

	   ppp unit receive-side lock. */

	for (p = list->next; p != (struct sk_buff *)list; p = p->next)

	skb_queue_walk(list, p) {

		if (seq_before(seq, p->sequence))

			break;

	}

	__skb_queue_before(list, p, skb);

}