[NETFILTER]: ip_tables: fix compat types

Use compat types and compat iterators when dealing with compat entries for
clarity. This doesn't actually make a difference for ip_tables, but is
needed for ip6_tables and arp_tables.

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