netfilter: only do skb_checksum_help on CHECKSUM_PARTIAL in ip6_queue

As we will set ip_summed to CHECKSUM_NONE when necessary in
ipq_mangle_ipv6, 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>