quota2: add a no-change mode

This commit adds an option to xt_quota2 called "no-change". The
effect of this option, when used, is that it will skip incrementing
or decrementing the quota counter, effectively providing a quota test
only.

The reason for implementing this is so that I could have a rule check
if quota is available for a rule in the PREROUTING tables, without
actually decrementing the amount of available quota. I only wanted to
decrement the amount of available quota in the FORWARD rule.
Otherwise, the first packet of every connection would be counted
twice.

extensions/xt_quota2.h

@@ -2,10 +2,11 @@
 #define _XT_QUOTA_H
 
 enum xt_quota_flags {
-	XT_QUOTA_INVERT = 1 << 0,
-	XT_QUOTA_GROW   = 1 << 1,
-	XT_QUOTA_PACKET = 1 << 2,
-	XT_QUOTA_MASK   = 0x7,
+	XT_QUOTA_INVERT    = 1 << 0,
+	XT_QUOTA_GROW      = 1 << 1,
+	XT_QUOTA_PACKET    = 1 << 2,
+	XT_QUOTA_NO_CHANGE = 1 << 3,
+	XT_QUOTA_MASK      = 0x0F,
 };
 
 struct xt_quota_counter;