skbuff: optimize the pull_pages code in __pskb_pull_tail()

In the pull_pages code block, if the first frag size > eat,
we can end the loop in advance to avoid extra copy.
Signed-off-by: NLin Zhang <xiaolou4617@gmail.com>
Acked-by: NEric Dumazet <edumazet@google.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/core/skbuff.c

@@ -1719,6 +1719,8 @@ void *__pskb_pull_tail(struct sk_buff *skb, int delta)
 			if (eat) {
 				skb_shinfo(skb)->frags[k].page_offset += eat;
 				skb_frag_size_sub(&skb_shinfo(skb)->frags[k], eat);
+				if (!i)
+					goto end;
 				eat = 0;
 			}
 			k++;
@@ -1726,6 +1728,7 @@ void *__pskb_pull_tail(struct sk_buff *skb, int delta)
 	}
 	skb_shinfo(skb)->nr_frags = k;
 
+end:
 	skb->tail     += delta;
 	skb->data_len -= delta;