platform/x86: thinkpad_acpi: Use strndup_user() in dispatch_proc_write()

Simplify the user copy code by using strndup_user().
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>

drivers/platform/x86/thinkpad_acpi.c

@@ -885,19 +885,11 @@ static ssize_t dispatch_proc_write(struct file *file,
 
 	if (!ibm || !ibm->write)
 		return -EINVAL;
-	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 = strndup_user(userbuf, PAGE_SIZE);
+	if (IS_ERR(kernbuf))
+		return PTR_ERR(kernbuf);
 
-	kernbuf[count] = 0;
 	ret = ibm->write(kernbuf);
 	if (ret == 0)
 		ret = count;