gro: Fix handling of complete checksums in IPv6

We need to perform skb_postpull_rcsum after pulling the IPv6
header in order to maintain the correctness of the complete
checksum.

This patch also adds a missing iph reload after pulling.

Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: David S. Miller <davem@davemloft.net>