[NETFILTER]: x_tables: netns propagation for /proc/net/*_tables_names

Propagate netns together with AF down to ->start/->next/->stop
iterators. Choose table based on netns and AF for showing.

Signed-off-by: Alexey Dobriyan <adobriyan@sw.ru>
Signed-off-by: Patrick McHardy <kaber@trash.net>
Signed-off-by: David S. Miller <davem@davemloft.net>