[NETFILTER]: ip_tables: account for struct ipt_entry/struct compat_ipt_entry size diff

Account for size differences when dumping entries or calculating the
entry positions. This doesn't actually make any difference for IPv4
since the structures have the same size, but its logically correct
and needed for IPv6.

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