[NETFILTER]: Clean up table initialization

- move arp_tables initial table structure definitions to arp_tables.h
  similar to ip_tables and ip6_tables

- use C99 initializers

- use initializer macros where possible

Signed-off-by: Patrick McHardy <kaber@trash.net>
Signed-off-by: David S. Miller <davem@davemloft.net>