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>

net/core/sock.c

@@ -2332,7 +2332,7 @@ static void __release_sock(struct sock *sk)
 			next = skb->next;
 			prefetch(next);
 			WARN_ON_ONCE(skb_dst_is_noref(skb));
-			skb->next = NULL;
+			skb_mark_not_on_list(skb);
 			sk_backlog_rcv(sk, skb);
 
 			cond_resched();