netfilter: xtables: deconstify struct xt_action_param for matches

In future, layer-3 matches will be an xt module of their own, and
need to set the fragoff and thoff fields. Adding more pointers would
needlessy increase memory requirements (esp. so for 64-bit, where
pointers are wider).

Signed-off-by: Jan Engelhardt <jengelh@medozas.de>

include/linux/netfilter/x_tables.h

@@ -288,7 +288,7 @@ struct xt_match {
 	   non-linear skb, using skb_header_pointer and
 	   skb_ip_make_writable. */
 	bool (*match)(const struct sk_buff *skb,
-		      const struct xt_action_param *);
+		      struct xt_action_param *);
 
 	/* Called when user tries to insert an entry of this type. */
 	int (*checkentry)(const struct xt_mtchk_param *);