netfilter: xt_cgroup: initialize info->priv in cgroup_mt_check_v1()

xt_cgroup_info_v1->priv is an internal pointer only used for kernel,
we should not trust what user-space provides.

Reported-by:  <syzbot+4fbcfcc0d2e6592bd641@syzkaller.appspotmail.com>
Fixes: c38c4597 ("netfilter: implement xt_cgroup cgroup2 path match")
Cc: Pablo Neira Ayuso <pablo@netfilter.org>
Signed-off-by: Cong Wang <xiyou.wangcong@gmail.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>