netfilter: arp_tables: init netns pointer in xt_tgdtor_param struct

An earlier commit (1b789577,
"netfilter: arp_tables: init netns pointer in xt_tgchk_param struct")
fixed missing net initialization for arptables, but turns out it was
incomplete.  We can get a very similar struct net NULL deref during
error unwinding:

general protection fault: 0000 [#1] PREEMPT SMP KASAN
RIP: 0010:xt_rateest_put+0xa1/0x440 net/netfilter/xt_RATEEST.c:77
 xt_rateest_tg_destroy+0x72/0xa0 net/netfilter/xt_RATEEST.c:175
 cleanup_entry net/ipv4/netfilter/arp_tables.c:509 [inline]
 translate_table+0x11f4/0x1d80 net/ipv4/netfilter/arp_tables.c:587
 do_replace net/ipv4/netfilter/arp_tables.c:981 [inline]
 do_arpt_set_ctl+0x317/0x650 net/ipv4/netfilter/arp_tables.c:1461

Also init the netns pointer in xt_tgdtor_param struct.

Fixes: add67461 ("netfilter: add struct net * to target parameters")
Reported-by:  <syzbot+91bdd8eece0f6629ec8b@syzkaller.appspotmail.com>
Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>