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/ipv6/ip6_output.c b/net/ipv6/ip6_output.c
index 16f200f..9a8934a 100644
--- a/net/ipv6/ip6_output.c
+++ b/net/ipv6/ip6_output.c
@@ -727,7 +727,7 @@ int ip6_fragment(struct net *net, struct sock *sk, struct sk_buff *skb,
 
 			skb = frag;
 			frag = skb->next;
-			skb->next = NULL;
+			skb_mark_not_on_list(skb);
 		}
 
 		kfree(tmp_hdr);