[NET]: Store skb->timestamp as offset to a base timestamp

Reduces skb size by 8 bytes on 64-bit.

Signed-off-by: Patrick McHardy <kaber@trash.net>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/net/ipv6/reassembly.c b/net/ipv6/reassembly.c
index 59e7c63..9d9e043 100644
--- a/net/ipv6/reassembly.c
+++ b/net/ipv6/reassembly.c
@@ -562,7 +562,7 @@
 	if (skb->dev)
 		fq->iif = skb->dev->ifindex;
 	skb->dev = NULL;
-	fq->stamp = skb->stamp;
+	skb_get_timestamp(skb, &fq->stamp);
 	fq->meat += skb->len;
 	atomic_add(skb->truesize, &ip6_frag_mem);
 
@@ -664,7 +664,7 @@
 
 	head->next = NULL;
 	head->dev = dev;
-	head->stamp = fq->stamp;
+	skb_set_timestamp(head, &fq->stamp);
 	head->nh.ipv6h->payload_len = htons(payload_len);
 
 	*skb_in = head;