netfilter: kill redundant check code in which setting ip_summed value

If the returned csum value is 0, We has set ip_summed with
CHECKSUM_UNNECESSARY flag in __skb_checksum_complete_head().

So this patch kills the check and changes to return to upper
caller directly.
Signed-off-by: NShan Wei <shanwei@cn.fujitsu.com>
Signed-off-by: NPatrick McHardy <kaber@trash.net>

net/ipv4/netfilter.c

@@ -212,9 +212,7 @@ static __sum16 nf_ip_checksum_partial(struct sk_buff *skb, unsigned int hook,
 		skb->csum = csum_tcpudp_nofold(iph->saddr, iph->daddr, protocol,
 					       skb->len - dataoff, 0);
 		skb->ip_summed = CHECKSUM_NONE;
-		csum = __skb_checksum_complete_head(skb, dataoff + len);
-		if (!csum)
-			skb->ip_summed = CHECKSUM_UNNECESSARY;
+		return __skb_checksum_complete_head(skb, dataoff + len);
 	}
 	return csum;
 }

net/ipv6/netfilter.c

@@ -151,9 +151,7 @@ static __sum16 nf_ip6_checksum_partial(struct sk_buff *skb, unsigned int hook,
 							 protocol,
 							 csum_sub(0, hsum)));
 		skb->ip_summed = CHECKSUM_NONE;
-		csum = __skb_checksum_complete_head(skb, dataoff + len);
-		if (!csum)
-			skb->ip_summed = CHECKSUM_UNNECESSARY;
+		return __skb_checksum_complete_head(skb, dataoff + len);
 	}
 	return csum;
 };