[NETFILTER]: ipt_CLUSTERIP: add compat code

Adjust structure size and don't expect pointers passed in from
userspace to be valid. Also replace an enum in an ABI structure
by a fixed size type.

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