net: Convert broute_net_ops, frame_filter_net_ops and frame_nat_net_ops

These pernet_operations use ebt_register_table() and
ebt_unregister_table() to act on the tables, which
are used as argument in ebt_do_table(), called from
ebtables hooks.

Since there are no net-related bridge packets in-flight,
when the init and exit methods are called, these
pernet_operations are safe to be executed in parallel
with any other.

Signed-off-by: Kirill Tkhai <ktkhai@virtuozzo.com>
Signed-off-by: David S. Miller <davem@davemloft.net>