[NETFILTER]: x_tables: per-netns xt_tables

In fact all we want is per-netns set of rules, however doing that will
unnecessary complicate routines such as ipt_hook()/ipt_do_table, so
make full xt_table array per-netns.

Every user stubbed with init_net for a while.

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>