[NETFILTER]: {ip,ip6}_tables: remove x_tables wrapper functions

Use the x_tables functions directly to make it better visible which
parts are shared between ip_tables and ip6_tables.

Signed-off-by: Jan Engelhardt <jengelh@gmx.de>
Signed-off-by: Patrick McHardy <kaber@trash.net>
Signed-off-by: David S. Miller <davem@davemloft.net>