child-sa: Do not install mark on inbound kernel SA

child-sa: Do not install mark on inbound kernel SA

The SA ID (src, dst, proto, spi) is unique on ingress.

As such, explicit inbound marking is not needed to match an SA.

On the other hand, requiring inbound SAs to use marks forces the
installation of a mechanism for marking traffic (e.g. iptables) based
on some criteria.

Defining the criteria becomes complicated, for example when required to
support multiple SAs from the same src, especially when traffic is UDP
encapsulated.

This commit removes the assignment of the child_sa mark_in to the inbound SA.

Policies can be arbitrated by existing means - e.g, via netfilter policy
matching or using VTI interfaces - without the need to classify the flows prior
to state matching.

Since the reqid allocator regards the mark value, there is no risk of matching
the wrong policy.

And as explicit marking was required for route-based VPN to work before this
change, it should not cause regressions in existing setups.

Closes strongswan/strongswan#59.