ip_frag: Remove some atomic ops

Instead of doing one atomic operation per frag, we can factorize them.
Signed-off-by: NEric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/ipv4/ip_fragment.c

@@ -556,7 +556,6 @@ static int ip_frag_reasm(struct ipq *qp, struct sk_buff *prev,
 
 	skb_shinfo(head)->frag_list = head->next;
 	skb_push(head, head->data - skb_network_header(head));
-	atomic_sub(head->truesize, &qp->q.net->mem);
 
 	for (fp=head->next; fp; fp = fp->next) {
 		head->data_len += fp->len;
@@ -566,8 +565,8 @@ static int ip_frag_reasm(struct ipq *qp, struct sk_buff *prev,
 		else if (head->ip_summed == CHECKSUM_COMPLETE)
 			head->csum = csum_add(head->csum, fp->csum);
 		head->truesize += fp->truesize;
-		atomic_sub(fp->truesize, &qp->q.net->mem);
 	}
+	atomic_sub(head->truesize, &qp->q.net->mem);
 
 	head->next = NULL;
 	head->dev = dev;

net/ipv6/reassembly.c

@@ -524,7 +524,6 @@ static int ip6_frag_reasm(struct frag_queue *fq, struct sk_buff *prev,
 	skb_shinfo(head)->frag_list = head->next;
 	skb_reset_transport_header(head);
 	skb_push(head, head->data - skb_network_header(head));
-	atomic_sub(head->truesize, &fq->q.net->mem);
 
 	for (fp=head->next; fp; fp = fp->next) {
 		head->data_len += fp->len;
@@ -534,8 +533,8 @@ static int ip6_frag_reasm(struct frag_queue *fq, struct sk_buff *prev,
 		else if (head->ip_summed == CHECKSUM_COMPLETE)
 			head->csum = csum_add(head->csum, fp->csum);
 		head->truesize += fp->truesize;
-		atomic_sub(fp->truesize, &fq->q.net->mem);
 	}
+	atomic_sub(head->truesize, &fq->q.net->mem);
 
 	head->next = NULL;
 	head->dev = dev;