netfilter: xt_hashlimit: fix namespace destroy path

recent_net_exit() is called before recent_mt_destroy() in the
destroy path of network namespaces. Make sure there are no entries
in the parent proc entry xt_recent before removing it.

Signed-off-by: Vitaly E. Lavrov <lve@guap.ru>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>