Fix a bug, where no SACK is sent when receiving a FORWARD-TSN or

I-FORWARD-TSN chunk before any DATA or I-DATA chunk.

Thanks to Julian Cordes for finding this problem and prividing
packetdrill scripts to reporduce the issue.

MFC after: 3 days

sys/netinet/sctp_input.c

@@ -5515,6 +5515,11 @@ __attribute__((noinline))
 					*offset = length;
 					return (NULL);
 				}
+				/*
+				 * For sending a SACK this looks like DATA
+				 * chunks.
+				 */
+				stcb->asoc.last_data_chunk_from = stcb->asoc.last_control_chunk_from;
 				sctp_handle_forward_tsn(stcb,
 				    (struct sctp_forward_tsn_chunk *)ch, &abort_flag, m, *offset);
 				if (abort_flag) {