USB: usbfs: compute urb->actual_length for isochronous

The USB kerneldoc says that the actual_length field "is read in
non-iso completion functions", but the usbfs driver uses it for all
URB types in processcompl().  Since not all of the host controller
drivers set actual_length for isochronous URBs, programs using usbfs
with some host controllers don't work properly.  For example, Minas
reports that a USB camera controlled by libusb doesn't work properly
with a dwc2 controller.

It doesn't seem worthwhile to change the HCDs and the documentation,
since the in-kernel USB class drivers evidently don't rely on
actual_length for isochronous transfers.  The easiest solution is for
usbfs to calculate the actual_length value for itself, by adding up
the lengths of the individual packets in an isochronous transfer.

Signed-off-by: Alan Stern <stern@rowland.harvard.edu>
CC: Minas Harutyunyan <Minas.Harutyunyan@synopsys.com>
Reported-and-tested-by: wlf <wulf@rock-chips.com>
CC: <stable@vger.kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>