net/mwan3: fix ping issue if last interface recovers from failure

Even though error was fixed the interface checks still fails, if last_resort
was set to blackhole or unreachable.

To fix this issue do not remove failure interface from iptables change on
down event.

Reported-by: Colby Whitney <colby.whitney@luxul.com>
Signed-off-by: Florian Eckert <fe@dev.tdt.de>
(cherry picked from commit 6d99b602fd)

net/mwan3/files/etc/hotplug.d/iface/15-mwan3

@@ -53,7 +53,6 @@ case "$ACTION" in
 	;;
 	ifdown)
 		mwan3_delete_iface_rules $INTERFACE
-		mwan3_delete_iface_iptables $INTERFACE
 		mwan3_delete_iface_route $INTERFACE
 		mwan3_delete_iface_ipset_entries $INTERFACE
 		mwan3_set_policies_iptables

net/mwan3/files/usr/sbin/mwan3

@@ -46,6 +46,7 @@ ifdown()
 		kill $(cat /var/run/mwan3track-$1.pid)
 		rm /var/run/mwan3track-$1.pid
 	fi
+	mwan3_delete_iface_iptables $1
 }
 
 ifup()