net: Use __skb_pagelen() directly in skb_cow_data()

In fact, skb_pagelen() - skb_headlen() is equal to __skb_pagelen(), use it
directly to avoid unnecessary skb_headlen() call.

Also fix the CHECK note of checkpatch.pl:
    Comparison to NULL could be written "!__pskb_pull_tail"
Signed-off-by: Miaohe Lin <linmiaohe@huawei.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/core/skbuff.c

@@ -4413,7 +4413,7 @@ int skb_cow_data(struct sk_buff *skb, int tailbits, struct sk_buff **trailer)
 	 * at the moment even if they are anonymous).
 	 */
 	if ((skb_cloned(skb) || skb_shinfo(skb)->nr_frags) &&
-	    __pskb_pull_tail(skb, skb_pagelen(skb)-skb_headlen(skb)) == NULL)
+	    !__pskb_pull_tail(skb, __skb_pagelen(skb)))
 		return -ENOMEM;
 
 	/* Easy case. Most of packets will go this way. */