skbuff: pass the result of data ksize to __build_skb_around

Avoid to call ksize again in __build_skb_around by passing
the result of data ksize to __build_skb_around

nginx stress test shows this change can reduce ksize cpu usage,
and give a little performance boost
Signed-off-by: NLi RongQing <lirongqing@baidu.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/core/skbuff.c

@@ -394,8 +394,9 @@ struct sk_buff *__alloc_skb(unsigned int size, gfp_t gfp_mask,
 {
 	struct kmem_cache *cache;
 	struct sk_buff *skb;
-	u8 *data;
+	unsigned int osize;
 	bool pfmemalloc;
+	u8 *data;
 
 	cache = (flags & SKB_ALLOC_FCLONE)
 		? skbuff_fclone_cache : skbuff_head_cache;
@@ -427,7 +428,8 @@ struct sk_buff *__alloc_skb(unsigned int size, gfp_t gfp_mask,
 	 * Put skb_shared_info exactly at the end of allocated zone,
 	 * to allow max possible filling before reallocation.
 	 */
-	size = SKB_WITH_OVERHEAD(ksize(data));
+	osize = ksize(data);
+	size = SKB_WITH_OVERHEAD(osize);
 	prefetchw(data + size);
 
 	/*
@@ -436,7 +438,7 @@ struct sk_buff *__alloc_skb(unsigned int size, gfp_t gfp_mask,
 	 * the tail pointer in struct sk_buff!
 	 */
 	memset(skb, 0, offsetof(struct sk_buff, tail));
-	__build_skb_around(skb, data, 0);
+	__build_skb_around(skb, data, osize);
 	skb->pfmemalloc = pfmemalloc;
 
 	if (flags & SKB_ALLOC_FCLONE) {