skbuff: add stub to help computing crc32c on SCTP packets

sctp_compute_checksum requires crc32c symbol (provided by libcrc32c), so
it can't be used in net core. Like it has been done previously with other
symbols (e.g. ipv6_dst_lookup), introduce a stub struct skb_checksum_ops
to allow computation of crc32c checksum in net core after sctp.ko (and thus
libcrc32c) has been loaded.

Signed-off-by: Davide Caratti <dcaratti@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/net/sctp/offload.c b/net/sctp/offload.c
index 4f5a2b5..b671984 100644
--- a/net/sctp/offload.c
+++ b/net/sctp/offload.c
@@ -98,6 +98,11 @@ static const struct net_offload sctp6_offload = {
 	},
 };
 
+static const struct skb_checksum_ops crc32c_csum_ops = {
+	.update  = sctp_csum_update,
+	.combine = sctp_csum_combine,
+};
+
 int __init sctp_offload_init(void)
 {
 	int ret;
@@ -110,6 +115,7 @@ int __init sctp_offload_init(void)
 	if (ret)
 		goto ipv4;
 
+	crc32c_csum_stub = &crc32c_csum_ops;
 	return ret;
 
 ipv4: