net: Add and use skb_mark_not_on_list().

An SKB is not on a list if skb->next is NULL.

Codify this convention into a helper function and use it
where we are dequeueing an SKB and need to mark it as such.

Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/net/sched/sch_cake.c b/net/sched/sch_cake.c
index c07c30b..dc53929 100644
--- a/net/sched/sch_cake.c
+++ b/net/sched/sch_cake.c
@@ -812,7 +812,7 @@
 
 	if (skb) {
 		flow->head = skb->next;
-		skb->next = NULL;
+		skb_mark_not_on_list(skb);
 	}
 
 	return skb;
@@ -1252,7 +1252,7 @@
 	else
 		flow->head = elig_ack->next;
 
-	elig_ack->next = NULL;
+	skb_mark_not_on_list(elig_ack);
 
 	return elig_ack;
 }
@@ -1675,7 +1675,7 @@
 
 		while (segs) {
 			nskb = segs->next;
-			segs->next = NULL;
+			skb_mark_not_on_list(segs);
 			qdisc_skb_cb(segs)->pkt_len = segs->len;
 			cobalt_set_enqueue_time(segs, now);
 			get_cobalt_cb(segs)->adjusted_len = cake_overhead(q,