usb: gadget: inode: switch over to memdup_user()

This patch fixes the following Coccinelle warning:

drivers/usb/gadget/inode.c:442:8-15: WARNING \
	opportunity for memdup_user
Signed-off-by: Felipe Balbi <balbi@ti.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/usb/gadget/inode.c

@@ -439,11 +439,9 @@ ep_write (struct file *fd, const char __user *buf, size_t len, loff_t *ptr)
 	/* FIXME writebehind for O_NONBLOCK and poll(), qlen = 1 */
 
 	value = -ENOMEM;
-	kbuf = kmalloc (len, GFP_KERNEL);
-	if (!kbuf)
-		goto free1;
-	if (copy_from_user (kbuf, buf, len)) {
-		value = -EFAULT;
+	kbuf = memdup_user(buf, len);
+	if (!kbuf) {
+		value = PTR_ERR(kbuf);
 		goto free1;
 	}
 
@@ -452,7 +450,6 @@ ep_write (struct file *fd, const char __user *buf, size_t len, loff_t *ptr)
 		data->name, len, (int) value);
 free1:
 	mutex_unlock(&data->lock);
-	kfree (kbuf);
 	return value;
 }