xtables: unify user chain add/flush for restore case

xtables: unify user chain add/flush for restore case

The idea here is to move the 'flush' decision into the core, rather than
have the decision in the frontend.

This will be required later when "generation id" is passed to kernel.
In this case, we might have to add the flush when re-trying the
transaction.

Signed-off-by: Florian Westphal <fw@strlen.de>
Acked-by: Pablo Neira Ayuso <pablo@netfilter.org>