[NETFILTER]: futher {ip,ip6,arp}_tables unification

This patch moves {ip,ip6,arp}t_entry_{match,target} definitions to
x_tables.h. This move simplifies code and future compatibility fixes.

Signed-off-by: Dmitry Mishin <dim@openvz.org>
Acked-off-by: Kirill Korotaev <dev@openvz.org>
Signed-off-by: Patrick McHardy <kaber@trash.net>
Signed-off-by: David S. Miller <davem@davemloft.net>