netfilter: fix calls to nf_ct_iterate_cleanup() with Linux >= 3.13

Signed-off-by: Jo-Philipp Wich <jow@openwrt.org>

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@42093 3c298f89-4303-0410-b956-a3cf2f4a3e73