ipvs: handle connections started by real-servers

When using LVS-NAT and SIP persistence-egine over UDP, the following
limitations are present with current implementation:

  1) To actually have load-balancing based on Call-ID header, you need to
     use one-packet-scheduling mode. But with one-packet-scheduling the
     connection is deleted just after packet is forwarded, so SIP responses
     coming from real-servers do not match any connection and SNAT is
     not applied.

  2) If you do not use "-o" option, IPVS behaves as normal UDP load
     balancer, so different SIP calls (each one identified by a different
     Call-ID) coming from the same ip-address/port go to the same
     real-server. So basically you don’t have load-balancing based on
     Call-ID as intended.

  3) Call-ID is not learned when a new SIP call is started by a real-server
     (inside-to-outside direction), but only in the outside-to-inside
     direction. This would be a general problem for all SIP servers acting
     as Back2BackUserAgent.

This patch aims to solve problems 1) and 3) while keeping OPS mode
mandatory for SIP-UDP, so that 2) is not a problem anymore.

The basic mechanism implemented is to make packets, that do not match any
existent connection but come from real-servers, create new connections
instead of let them pass without any effect.
When such packets pass through ip_vs_out(), if their source ip address and
source port match a configured real-server, a new connection is
automatically created in the same way as it would have happened if the
packet had come from outside-to-inside direction. A new connection template
is created too if the virtual-service is persistent and there is no
matching connection template found. The new connection automatically
created, if the service had "-o" option, is an OPS connection that lasts
only the time to forward the packet, just like it happens on the
ingress side.

The main part of this mechanism is implemented inside a persistent-engine
specific callback (at the moment only SIP persistent engine exists) and
is triggered only for UDP packets, since connection oriented protocols, by
using different set of ports (typically ephemeral ports) to open new
outgoing connections, should not need this feature.

The following requisites are needed for automatic connection creation; if
any is missing the packet simply goes the same way as before.
a) virtual-service is not fwmark based (this is because fwmark services
   do not store address and port of the virtual-service, required to
   build the connection data).
b) virtual-service and real-servers must not have been configured with
   omitted port (this is again to have all data to create the connection).

Signed-off-by: Marco Angaroni <marcoangaroni@gmail.com>
Acked-by: Julian Anastasov <ja@ssi.bg>
Signed-off-by: Simon Horman <horms@verge.net.au>