sctp: Use software crc32 checksum when xfrm transform will happen.

igb/ixgbe have hardware sctp checksum support, when this feature is enabled
and also IPsec is armed to protect sctp traffic, ugly things happened as
xfrm_output checks CHECKSUM_PARTIAL to do checksum operation(sum every thing
up and pack the 16bits result in the checksum field). The result is fail
establishment of sctp communication.

Cc: Neil Horman <nhorman@tuxdriver.com>
Cc: Steffen Klassert <steffen.klassert@secunet.com>
Signed-off-by: Fan Du <fan.du@windriver.com>
Signed-off-by: Vlad Yasevich <vyasevich@gmail.com>
Acked-by: Neil Horman <nhorman@tuxdriver.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/sctp/output.c

@@ -536,7 +536,8 @@ int sctp_packet_transmit(struct sctp_packet *packet)
 	 * by CRC32-C as described in <draft-ietf-tsvwg-sctpcsum-02.txt>.
 	 */
 	if (!sctp_checksum_disable) {
-		if (!(dst->dev->features & NETIF_F_SCTP_CSUM)) {
+		if (!(dst->dev->features & NETIF_F_SCTP_CSUM) ||
+		    (dst_xfrm(dst) != NULL)) {
 			__u32 crc32 = sctp_start_cksum((__u8 *)sh, cksum_buf_len);
 
 			/* 3) Put the resultant value into the checksum field in the