Using the unified way to compare pointers and NULL, which cleans the static
warning.

eg:
	if (skb == NULL) --> if (!skb）
	if (skb != NULL) --> if (skb)
Signed-off-by: Guofeng Yue <yueguofeng@hisilicon.com>
Signed-off-by: Haoyue Xu <xuhaoyue1@hisilicon.com>
Signed-off-by: David S. Miller <davem@davemloft.net>