isdn: mISDN: socket: Fix set-but-unused variables.

The variable 'len' is set but unused in data_sock_getsockopt().

The code should use 'len' to validate that the user's socket option is
indeed the right size.
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/isdn/mISDN/socket.c

@@ -457,6 +457,9 @@ static int data_sock_getsockopt(struct socket *sock, int level, int optname,
 	if (get_user(len, optlen))
 		return -EFAULT;
 
+	if (len != sizeof(char))
+		return -EINVAL;
+
 	switch (optname) {
 	case MISDN_TIME_STAMP:
 		if (_pms(sk)->cmask & MISDN_TIME_STAMP)