platform/x86: thinkpad_acpi: Switch to memdup_user_nul() helper

Use memdup_user_nul() helper instead of open-coding to simplify the code.
Signed-off-by: Ruan Jinjie <ruanjinjie@huawei.com>
Link: https://lore.kernel.org/r/20230810122012.2110410-1-ruanjinjie@huawei.comReviewed-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>

drivers/platform/x86/thinkpad_acpi.c

@@ -913,16 +913,9 @@ static ssize_t dispatch_proc_write(struct file *file,
 	if (count > PAGE_SIZE - 1)
 		return -EINVAL;
 
-	kernbuf = kmalloc(count + 1, GFP_KERNEL);
-	if (!kernbuf)
-		return -ENOMEM;
-
-	if (copy_from_user(kernbuf, userbuf, count)) {
-		kfree(kernbuf);
-		return -EFAULT;
-	}
-
-	kernbuf[count] = 0;
+	kernbuf = memdup_user_nul(userbuf, count);
+	if (IS_ERR(kernbuf))
+		return PTR_ERR(kernbuf);
 	ret = ibm->write(kernbuf);
 	if (ret == 0)
 		ret = count;