netfilter: nf_ct_sctp: minimal multihoming support

Currently nf_conntrack_proto_sctp module handles only packets between
primary addresses used to establish the connection. Any packets between
secondary addresses are classified as invalid so that usual firewall
configurations drop them. Allowing HEARTBEAT and HEARTBEAT-ACK chunks to
establish a new conntrack would allow traffic between secondary
addresses to pass through. A more sophisticated solution based on the
addresses advertised in the initial handshake (and possibly also later
dynamic address addition and removal) would be much harder to implement.
Moreover, in general we cannot assume to always see the initial
handshake as it can be routed through a different path.

The patch adds two new conntrack states:

  SCTP_CONNTRACK_HEARTBEAT_SENT  - a HEARTBEAT chunk seen but not acked
  SCTP_CONNTRACK_HEARTBEAT_ACKED - a HEARTBEAT acked by HEARTBEAT-ACK

State transition rules:

- HEARTBEAT_SENT responds to usual chunks the same way as NONE (so that
  the behaviour changes as little as possible)
- HEARTBEAT_ACKED responds to usual chunks the same way as ESTABLISHED
  does, except the resulting state is HEARTBEAT_ACKED rather than
  ESTABLISHED
- previously existing states except NONE are preserved when HEARTBEAT or
  HEARTBEAT-ACK is seen
- NONE (in the initial direction) changes to HEARTBEAT_SENT on HEARTBEAT
  and to CLOSED on HEARTBEAT-ACK
- HEARTBEAT_SENT changes to HEARTBEAT_ACKED on HEARTBEAT-ACK in the
  reply direction
- HEARTBEAT_SENT and HEARTBEAT_ACKED are preserved on HEARTBEAT and
  HEARTBEAT-ACK otherwise

Normally, vtag is set from the INIT chunk for the reply direction and
from the INIT-ACK chunk for the originating direction (i.e. each of
these defines vtag value for the opposite direction). For secondary
conntracks, we can't rely on seeing INIT/INIT-ACK and even if we have
seen them, we would need to connect two different conntracks. Therefore
simplified logic is applied: vtag of first packet in each direction
(HEARTBEAT in the originating and HEARTBEAT-ACK in reply direction) is
saved and all following packets in that direction are compared with this
saved value. While INIT and INIT-ACK define vtag for the opposite
direction, vtags extracted from HEARTBEAT and HEARTBEAT-ACK are always
for their direction.

Default timeout values for new states are

  HEARTBEAT_SENT: 30 seconds (default hb_interval)
  HEARTBEAT_ACKED: 210 seconds (hb_interval * path_max_retry + max_rto)

(We cannot expect to see the shutdown sequence so that, unlike
ESTABLISHED, the HEARTBEAT_ACKED timeout shouldn't be too long.)

Signed-off-by: Michal Kubecek <mkubecek@suse.cz>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>