netfilter: xtables: combine struct xt_match_param and xt_target_param

The structures carried - besides match/target - almost the same data.
It is possible to combine them, as extensions are evaluated serially,
and so, the callers end up a little smaller.

  text  data  bss  filename
-15318   740  104  net/ipv4/netfilter/ip_tables.o
+15286   740  104  net/ipv4/netfilter/ip_tables.o
-15333   540  152  net/ipv6/netfilter/ip6_tables.o
+15269   540  152  net/ipv6/netfilter/ip6_tables.o

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