ptp: oops in ptp_ioctl()

If we pass ERR_PTR(-EFAULT) to kfree() then it's going to oops.

Fixes: 2ece068e1b1d ('ptp: use memdup_user().')
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Acked-by: Richard Cochran <richardcochran@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/ptp/ptp_chardev.c

@@ -211,6 +211,7 @@ long ptp_ioctl(struct posix_clock *pc, unsigned int cmd, unsigned long arg)
 		sysoff = memdup_user((void __user *)arg, sizeof(*sysoff));
 		if (IS_ERR(sysoff)) {
 			err = PTR_ERR(sysoff);
+			sysoff = NULL;
 			break;
 		}
 		if (sysoff->n_samples > PTP_MAX_SAMPLES) {