netfilter: xt_RATEEST: reject non-null terminated string from userspace

syzbot reports:
detected buffer overflow in strlen
[..]
Call Trace:
 strlen include/linux/string.h:325 [inline]
 strlcpy include/linux/string.h:348 [inline]
 xt_rateest_tg_checkentry+0x2a5/0x6b0 net/netfilter/xt_RATEEST.c:143

strlcpy assumes src is a c-string. Check info->name before its used.

Reported-by: syzbot+e86f7c428c8c50db65b4@syzkaller.appspotmail.com
Fixes: 5859034d7e ("[NETFILTER]: x_tables: add RATEEST target")
Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>

net/netfilter/xt_RATEEST.c

@@ -115,6 +115,9 @@ static int xt_rateest_tg_checkentry(const struct xt_tgchk_param *par)
 	} cfg;
 	int ret;
 
+	if (strnlen(info->name, sizeof(est->name)) >= sizeof(est->name))
+		return -ENAMETOOLONG;
+
 	net_get_random_once(&jhash_rnd, sizeof(jhash_rnd));
 
 	mutex_lock(&xn->hash_lock);