usbmon: correct length for isochronous

Usually the usbmon returns the amount of data specified in
urb->transfer_buffer_length for output submissions and urb->actual_length
for input callbacks. However, for Isochronous input transfers, this is
not enough, since the returned data buffer may contain "holes".

One easy way to fix this is to use urb->transfer_buffer_length,
but this often transfers a whole lot of unused data, so we find
how much was actually used instead.

Original patch by Márton Németh. See also kernel bug 22182.

Signed-off-by: Pete Zaitcev <zaitcev@redhat.com>
Signed-off-by: Márton Németh <nm127@freemail.hu>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>