usbdevfs: Fix broken scatter-gather transfer

The handling of large output bulk transfers is broken; the same user
page is read over and over again. Fixed with this patch.

Cc: stable <stable@vger.kernel.org>
Acked-by: Peter Stuge <peter@stuge.se>
Acked-by: Hans de Goede <hdegoede@redhat.com>
Acked-by: Alan Stern <stern@rowland.harvard.edu>
Signed-off-by: Henrik Rydberg <rydberg@euromail.se>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>