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/ipv4/ip_input.c

@@ -535,7 +535,7 @@ static void ip_sublist_rcv_finish(struct list_head *head)
 		/* Handle ip{6}_forward case, as sch_direct_xmit have
 		 * another kind of SKB-list usage (see validate_xmit_skb_list)
 		 */
-		skb->next = NULL;
+		skb_mark_not_on_list(skb);
 		dst_input(skb);
 	}
 }