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: NCong Wang <xiyou.wangcong@gmail.com>
Signed-off-by: NPablo Neira Ayuso <pablo@netfilter.org>

net/netfilter/xt_cgroup.c

@@ -52,6 +52,7 @@ static int cgroup_mt_check_v1(const struct xt_mtchk_param *par)
 		return -EINVAL;
 	}
 
+	info->priv = NULL;
 	if (info->has_path) {
 		cgrp = cgroup_get_from_path(info->path);
 		if (IS_ERR(cgrp)) {