kernel: net_sched: fix a NULL pointer deref in ipt action

kernel: net_sched: fix a NULL pointer deref in ipt action

The net pointer in struct xt_tgdtor_param is not explicitly
initialized therefore is still NULL when dereferencing it.
So we have to find a way to pass the correct net pointer to
ipt_destroy_target().

The best way I find is just saving the net pointer inside the per
netns struct tcf_idrinfo, which could make this patch smaller.

Fixes: 0c66dc1ea3f0 ("netfilter: conntrack: register hooks in netns when needed by ruleset")
Reported-and-tested-by: Tony Ambardar <itugrok@xxxxxxxxx>
Cc: Jamal Hadi Salim <jhs@xxxxxxxxxxxx>
Cc: Jiri Pirko <jiri@xxxxxxxxxxx>
Signed-off-by: Cong Wang <xiyou.wangcong@gmail.com>
[Backport for kernel v4.19 and v4.14]
Link: https://bugzilla.kernel.org/show_bug.cgi?id=204681]
Signed-off-by: Tony Ambardar <itugrok@yahoo.com>