s390 keyboard.c: don't open-code strndup_user()

... especially not with off-by-ones (strnlen_user() already includes NUL
into its count).

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

drivers/s390/char/keyboard.c

@@ -433,12 +433,7 @@ do_kdgkb_ioctl(struct kbd_data *kbd, struct kbsentry __user *u_kbs,
 	case KDSKBSENT:
 		if (!perm)
 			return -EPERM;
-		len = strnlen_user(u_kbs->kb_string, sizeof(u_kbs->kb_string));
-		if (!len)
-			return -EFAULT;
-		if (len > sizeof(u_kbs->kb_string))
-			return -EINVAL;
-		p = memdup_user_nul(u_kbs->kb_string, len);
+		p = strndup_user(u_kbs->kb_string, sizeof(u_kbs->kb_string));
 		if (IS_ERR(p))
 			return PTR_ERR(p);
 		kfree(kbd->func_table[kb_func]);