netfilter: xtables: remove almost-unused xt_match_param.data member

This member is taking up a "long" per match, yet is only used by one
module out of the roughly 90 modules, ip6t_hbh. ip6t_hbh can be
restructured a little to accomodate for the lack of the .data member.
This variant uses checking the par->match address, which should avoid
having to add two extra functions, including calls, i.e.

(hbh_mt6: call hbhdst_mt6(skb, par, NEXTHDR_OPT),
dst_mt6: call hbhdst_mt6(skb, par, NEXTHDR_DEST))

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

include/linux/netfilter/x_tables.h

@@ -315,9 +315,6 @@ struct xt_match {
 	/* Set this to THIS_MODULE if you are a module, otherwise NULL */
 	struct module *me;
 
-	/* Free to use by each match */
-	unsigned long data;
-
 	const char *table;
 	unsigned int matchsize;
 #ifdef CONFIG_COMPAT