netfilter: nf_tables: flow offload expression

Add new instruction for the nf_tables VM that allows us to specify what
flows are offloaded into a given flow table via name. This new
instruction creates the flow entry and adds it to the flow table.

Only established flows, ie. we have seen traffic in both directions, are
added to the flow table. You can still decide to offload entries at a
later stage via packet counting or checking the ct status in case you
want to offload assured conntracks.

This new extension depends on the conntrack subsystem.

Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>