netfilter: only do skb_checksum_help on CHECKSUM_PARTIAL in ip_queue

While doing yet another audit on ip_summed I noticed ip_queue
calling skb_checksum_help unnecessarily.  As we will set ip_summed
to CHECKSUM_NONE when necessary in ipq_mangle_ipv4, there is no
need to zap CHECKSUM_COMPLETE in ipq_build_packet_message.

Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: Patrick McHardy <kaber@trash.net>