s390/tape: Use memdup_user() rather than duplicating its implementation

Reuse existing functionality from memdup_user() instead of keeping
duplicate source code.

This issue was detected by using the Coccinelle software.
Signed-off-by: Markus Elfring <elfring@users.sourceforge.net>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>

drivers/s390/char/tape_3590.c

@@ -312,15 +312,10 @@ static int tape_3592_ioctl_kekl_set(struct tape_device *device,
 		return -ENOSYS;
 	if (!crypt_enabled(device))
 		return -EUNATCH;
-	ext_kekls = kmalloc(sizeof(*ext_kekls), GFP_KERNEL);
-	if (!ext_kekls)
-		return -ENOMEM;
-	if (copy_from_user(ext_kekls, (char __user *)arg, sizeof(*ext_kekls))) {
-		rc = -EFAULT;
-		goto out;
-	}
+	ext_kekls = memdup_user((char __user *)arg, sizeof(*ext_kekls));
+	if (IS_ERR(ext_kekls))
+		return PTR_ERR(ext_kekls);
 	rc = tape_3592_kekl_set(device, ext_kekls);
-out:
 	kfree(ext_kekls);
 	return rc;
 }