iptables: fix undersized deletion mask creation

iptables: fix undersized deletion mask creation

The mask created for the -D rulespec is simply too small.
xtables_targets points to whatever target has last been loaded, so
xtables_targets->size is quite almost wrong, as we need to use the
size of the target for the specific rule that is about to be deleted.

This bug existed ever since iptables history is tracked, and requires
certain circumstances to be visible, where the deletion operation is
one. Furthermore, multiple userspace target extensions must have been
loaded, and a target B whose .size is smaller than the target A of
the rule we are about to delete must have been loaded more recently
than target A. The minimal testcase is (rule 60007 gets wrongly
removed)

	*nat
	-F
	-X
	-A POSTROUTING -p udp -j SNAT --to 192.168.1.1:60007
	-A POSTROUTING -p udp -j SNAT --to 192.168.1.1:60008
	-A POSTROUTING -p udp -j CONNMARK --set-mark 0
	-D POSTROUTING -p udp -j SNAT --to 192.168.1.1:60008
	COMMIT

References: http://bugzilla.netfilter.org/show_bug.cgi?id=606
Signed-off-by: Jan Engelhardt <jengelh@medozas.de>