netfilter: flowtable: bail out if forward path cannot be discovered

netfilter: flowtable: bail out if forward path cannot be discovered

If forward path discovery fails for any reason or netdevice is not
registered for this flowtable, then bail out to classic forwarding path
rather than providing incomplete forwarding path.

Update the existing forward path parser functions to report an error
so the flow_offload expressions gives up on setting up the flowtable
entry.

Link: https://sashiko.dev/#/patchset/20260607094954.48892-15-pablo%40netfilter.org?part=14
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>