l2tp: fix incorrect parameter validation in the pppol2tp_getsockopt() function

The 'len' variable can't be negative when assigned the result of
'min_t' because all 'min_t' parameters are cast to unsigned int,
and then the minimum one is chosen.

To fix the logic, check 'len' as read from 'optlen',
where the types of relevant variables are (signed) int.

Fixes: 3557baab ("[L2TP]: PPP over L2TP driver core")
Reviewed-by: Tom Parkin <tparkin@katalix.com>
Signed-off-by: Gavrilov Ilia <Ilia.Gavrilov@infotecs.ru>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/l2tp/l2tp_ppp.c

@@ -1356,11 +1356,11 @@ static int pppol2tp_getsockopt(struct socket *sock, int level, int optname,
 	if (get_user(len, optlen))
 		return -EFAULT;
 
-	len = min_t(unsigned int, len, sizeof(int));
-
 	if (len < 0)
 		return -EINVAL;
 
+	len = min_t(unsigned int, len, sizeof(int));
+
 	err = -ENOTCONN;
 	if (!sk->sk_user_data)
 		goto end;