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>