[NETFILTER]: xt_hashlimit: reduce overhead without IPv6

This patch generalizes the (CONFIG_IP6_NF_IPTABLES || CONFIG_IP6_NF_IPTABLES_MODULE)
test done in hashlimit_init_dst() to all the xt_hashlimit module.

This permits a size reduction of "struct dsthash_dst". This saves memory and
cpu for IPV4 only hosts.

Signed-off-by: Eric Dumazet <dada1@cosmosbay.com>
Signed-off-by: Patrick McHardy <kaber@trash.net>
Signed-off-by: David S. Miller <davem@davemloft.net>