netfilter: flowtable: move path discovery infrastructure to its own file

netfilter: flowtable: move path discovery infrastructure to its own file

This file contains the path discovery that is run from the forward chain
for the packet offloading the flow into the flowtable. This consists
of a series of calls to dev_fill_forward_path() for each device stack.

More topologies may be supported in the future, so move this code to its
own file to separate it from the nftables flow_offload expression.

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