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>

net/netfilter/nf_nat_proto_sctp.c

@@ -47,7 +47,10 @@ sctp_manip_pkt(struct sk_buff *skb,
 		hdr->dest = tuple->dst.u.sctp.port;
 	}
 
-	hdr->checksum = sctp_compute_cksum(skb, hdroff);
+	if (skb->ip_summed != CHECKSUM_PARTIAL) {
+		hdr->checksum = sctp_compute_cksum(skb, hdroff);
+		skb->ip_summed = CHECKSUM_NONE;
+	}
 
 	return true;
 }