usbcore/driver: Fix incorrect downcast

usbcore/driver: Fix incorrect downcast

This commit resolves a minor bug in the selection/discovery of more
specific USB device drivers for devices that are currently bound to
generic USB device drivers.

The bug is related to the way a candidate USB device driver is
compared against the generic USB device driver. The code in
is_dev_usb_generic_driver() assumes that the device driver in question
is a USB device driver by calling to_usb_device_driver(dev->driver)
to downcast; however I have observed that this assumption is not always
true, through code instrumentation.

This commit avoids the incorrect downcast altogether by comparing
the USB device's driver (i.e., dev->driver) to the generic USB
device driver directly. This method was suggested by Alan Stern.

This bug was found while investigating Andrey Konovalov's report
indicating usbip device driver misbehaviour with the recently merged
generic USB device driver selection feature. The report is linked
below.

Fixes: d5643d2249b2 ("USB: Fix device driver race")
Cc: <stable@vger.kernel.org> # 5.8
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Alan Stern <stern@rowland.harvard.edu>
Cc: Bastien Nocera <hadess@hadess.net>
Cc: Shuah Khan <shuah@kernel.org>
Cc: Valentina Manea <valentina.manea.m@gmail.com>
Cc: <syzkaller@googlegroups.com>
Tested-by: Andrey Konovalov <andreyknvl@google.com>
Signed-off-by: M. Vefa Bicakci <m.v.b@runbox.com>
Link: https://lore.kernel.org/r/20200922110703.720960-4-m.v.b@runbox.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>