net: ipvs: sctp: do not recalc sctp csum when ports didn't change

Unlike UDP or TCP, we do not take the pseudo-header into
account in SCTP checksums. So in case port mapping is the
very same, we do not need to recalculate the whole SCTP
checksum in software, which is very expensive.

Also, similarly as in TCP, take into account when a private
helper mangled the packet. In that case, we also need to
recalculate the checksum even if ports might be same.

Thanks for feedback regarding skb->ip_summed checks from
Julian Anastasov; here's a discussion on these checks for
snat and dnat:

* For snat_handler(), we can see CHECKSUM_PARTIAL from
  virtual devices, and from LOCAL_OUT, otherwise it
  should be CHECKSUM_UNNECESSARY. In general, in snat it
  is more complex. skb contains the original route and
  ip_vs_route_me_harder() can change the route after
  snat_handler. So, for locally generated replies from
  local server we can not preserve the CHECKSUM_PARTIAL
  mode. It is an chicken or egg dilemma: snat_handler
  needs the device after rerouting (to check for
  NETIF_F_SCTP_CSUM), while ip_route_me_harder() wants
  the snat_handler() to put the new saddr for proper
  rerouting.

* For dnat_handler(), we should not see CHECKSUM_COMPLETE
  for SCTP, in fact the small set of drivers that support
  SCTP offloading return CHECKSUM_UNNECESSARY on correctly
  received SCTP csum. We can see CHECKSUM_PARTIAL from
  local stack or received from virtual drivers. The idea is
  that SCTP decides to avoid csum calculation if hardware
  supports offloading. IPVS can change the device after
  rerouting to real server but we can preserve the
  CHECKSUM_PARTIAL mode if the new device supports
  offloading too. This works because skb dst is changed
  before dnat_handler and we see the new device. So, checks
  in the 'if' part will decide whether it is ok to keep
  CHECKSUM_PARTIAL for the output. If the packet was with
  CHECKSUM_NONE, hence we deal with unknown checksum. As we
  recalculate the sum for IP header in all cases, it should
  be safe to use CHECKSUM_UNNECESSARY. We can forward wrong
  checksum in this case (without cp->app). In case of
  CHECKSUM_UNNECESSARY, the csum was valid on receive.

Signed-off-by: Daniel Borkmann <dborkman@redhat.com>
Signed-off-by: Julian Anastasov <ja@ssi.bg>
Signed-off-by: Simon Horman <horms@verge.net.au>

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