mwan3: add sleep to release ipset reference

It turns out that under high system load, ipsets cannot be deleted. This
is because there is still a reference in iptables. A short sleep should
give the system time to clean this up.

Signed-off-by: Florian Eckert <fe@dev.tdt.de>

net/mwan3/files/etc/init.d/mwan3

@@ -91,6 +91,8 @@ stop_service() {
 		} | $IPTR
 	done
 
+	# Needed for the firewall backend to release the ipsets reference
+	sleep 2
 	for ipset in $($IPS -n list | grep mwan3_); do
 		$IPS -q destroy $ipset
 	done