runqemu-ifdown: clean up the remaining iptables rules

The iptables rules for the tap interface are added by runqemu-ifup
everytime we use runqemu to start a qemu target. But it's not cleaned
up when runqemu exits.

This patch cleans up the remaining iptables rules for the tap interface
in runqemu-ifdown.

[YOCTO #5047]

(From OE-Core rev: ef38a0aed35357d035ca587162158cd2f55b958f)

Signed-off-by: Chen Qi <Qi.Chen@windriver.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

scripts/runqemu-ifdown

@@ -50,3 +50,17 @@ if [ ! -e "$TUNCTL" ]; then
 fi
 
 $TUNCTL -d $TAP
+
+# cleanup the remaining iptables rules
+IPTABLES=`which iptables 2> /dev/null`
+if [ "x$IPTABLES" = "x" ]; then
+	IPTABLES=/sbin/iptables
+fi
+if [ ! -x "$IPTABLES" ]; then
+	echo "$IPTABLES cannot be executed"
+	exit 1
+fi
+n=$[ (`echo $TAP | sed 's/tap//'` * 2) + 1 ]
+dest=$[ (`echo $TAP | sed 's/tap//'` * 2) + 2 ]
+$IPTABLES -D POSTROUTING -t nat -j MASQUERADE -s 192.168.7.$n/32
+$IPTABLES -D POSTROUTING -t nat -j MASQUERADE -s 192.168.7.$dest/32