usb: gadget: fix check in sync read from ep in gadgetfs

When reading synchronously from a non-zero endpoint, gadgetfs will
return -EFAULT even if the read succeeds, due to a bad check of the
copy_to_iter() return value.

This fix compares the return value of copy_to_iter to the amount of
bytes that was passed, and only fails if they are not the same.
Signed-off-by: Binyamin Sharet <s.binyamin@gmail.com>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>

drivers/usb/gadget/legacy/inode.c

@@ -606,7 +606,7 @@ ep_read_iter(struct kiocb *iocb, struct iov_iter *to)
 	}
 	if (is_sync_kiocb(iocb)) {
 		value = ep_io(epdata, buf, len);
-		if (value >= 0 && copy_to_iter(buf, value, to))
+		if (value >= 0 && (copy_to_iter(buf, value, to) != value))
 			value = -EFAULT;
 	} else {
 		struct kiocb_priv *priv = kzalloc(sizeof *priv, GFP_KERNEL);