l2tp: fix set but not used variable

GCC complains:

net/l2tp/l2tp_ppp.c: In function ‘pppol2tp_ioctl’:
net/l2tp/l2tp_ppp.c:1073:6: warning: variable ‘val’ set but not used [-Wunused-but-set-variable]
  int val;
      ^~~
Signed-off-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Reviewed-by: Dirk van der Merwe <dirk.vandermerwe@netronome.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/l2tp/l2tp_ppp.c

@@ -1070,7 +1070,6 @@ static int pppol2tp_ioctl(struct socket *sock, unsigned int cmd,
 {
 	struct pppol2tp_ioc_stats stats;
 	struct l2tp_session *session;
-	int val;
 
 	switch (cmd) {
 	case PPPIOCGMRU:
@@ -1097,7 +1096,7 @@ static int pppol2tp_ioctl(struct socket *sock, unsigned int cmd,
 		if (!session->session_id && !session->peer_session_id)
 			return -ENOSYS;
 
-		if (get_user(val, (int __user *)arg))
+		if (!access_ok((int __user *)arg, sizeof(int)))
 			return -EFAULT;
 		break;