s390/keyboard: use memdup_user_nul()

Use memdup_user_nul to duplicate a memory region from user-space
to kernel-space and terminate with a NULL, instead of open coding
using kmalloc + copy_from_user and explicitly NULL terminating.
Signed-off-by: Muhammad Falak R Wani <falakreyaz@gmail.com>
[heiko.carstens@de.ibm.com: remove comment]
Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>

drivers/s390/char/keyboard.c

@@ -438,18 +438,9 @@ do_kdgkb_ioctl(struct kbd_data *kbd, struct kbsentry __user *u_kbs,
 			return -EFAULT;
 		if (len > sizeof(u_kbs->kb_string))
 			return -EINVAL;
-		p = kmalloc(len, GFP_KERNEL);
-		if (!p)
-			return -ENOMEM;
-		if (copy_from_user(p, u_kbs->kb_string, len)) {
-			kfree(p);
-			return -EFAULT;
-		}
-		/*
-		 * Make sure the string is terminated by 0. User could have
-		 * modified it between us running strnlen_user() and copying it.
-		 */
-		p[len - 1] = 0;
+		p = memdup_user_nul(u_kbs->kb_string, len);
+		if (IS_ERR(p))
+			return PTR_ERR(p);
 		kfree(kbd->func_table[kb_func]);
 		kbd->func_table[kb_func] = p;
 		break;