netfilter: conntrack: add sctp DATA_SENT state

SCTP conntrack currently assumes that the SCTP endpoints will
probe secondary paths using HEARTBEAT before sending traffic.

But, according to RFC 9260, SCTP endpoints can send any traffic
on any of the confirmed paths after SCTP association is up.
SCTP endpoints that sends INIT will confirm all peer addresses
that upper layer configures, and the SCTP endpoint that receives
COOKIE_ECHO will only confirm the address it sent the INIT_ACK to.

So, we can have a situation where the INIT sender can start to
use secondary paths without the need to send HEARTBEAT. This patch
allows DATA/SACK packets to create new connection tracking entry.

A new state has been added to indicate that a DATA/SACK chunk has
been seen in the original direction - SCTP_CONNTRACK_DATA_SENT.
State transitions mostly follows the HEARTBEAT_SENT, except on
receiving HEARTBEAT/HEARTBEAT_ACK/DATA/SACK in the reply direction.

State transitions in original direction:
- DATA_SENT behaves similar to HEARTBEAT_SENT for all chunks,
   except that it remains in DATA_SENT on receving HEARTBEAT,
   HEARTBEAT_ACK/DATA/SACK chunks
State transitions in reply direction:
- DATA_SENT behaves similar to HEARTBEAT_SENT for all chunks,
   except that it moves to HEARTBEAT_ACKED on receiving
   HEARTBEAT/HEARTBEAT_ACK/DATA/SACK chunks

Note: This patch still doesn't solve the problem when the SCTP
endpoint decides to use primary paths for association establishment
but uses a secondary path for association shutdown. We still have
to depend on timeout for connections to expire in such a case.

Signed-off-by: Sriram Yagnaraman <sriram.yagnaraman@est.tech>
Reviewed-by: Marcelo Ricardo Leitner <marcelo.leitner@gmail.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>