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: Shan Wei <shanwei@cn.fujitsu.com>
Signed-off-by: Patrick McHardy <kaber@trash.net>