BB: netfilter: support /proc conntrack flushing of specific ip addresses

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

Backport of r42092

git-svn-id: svn://svn.openwrt.org/openwrt/branches/barrier_breaker@42094 3c298f89-4303-0410-b956-a3cf2f4a3e73