netfilter: nat: skip checksum on offload SCTP packets

SCTP GSO and hardware can do CRC32c computation after netfilter processing,
so we can avoid calling sctp_compute_checksum() on skb if skb->ip_summed
is equal to CHECKSUM_PARTIAL. Moreover, set skb->ip_summed to CHECKSUM_NONE
when the NAT code computes the CRC, to prevent offloaders from computing
it again (on ixgbe this resulted in a transmission with wrong L4 checksum).

Signed-off-by: Davide Caratti <dcaratti@redhat.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>