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/rxrpc/input.c

@@ -259,7 +259,7 @@ static void rxrpc_rotate_tx_window(struct rxrpc_call *call, rxrpc_seq_t to,
 	while (list) {
 		skb = list;
 		list = skb->next;
-		skb->next = NULL;
+		skb_mark_not_on_list(skb);
 		rxrpc_free_skb(skb, rxrpc_skb_tx_freed);
 	}
 }