netfilter: ebtables: use net_generic infra

ebtables currently uses net->xt.tables[BRIDGE], but upcoming
patch will move net->xt.tables away from struct net.

To avoid exposing x_tables internals to ebtables, use a private list
instead.

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