USB: dummy-hcd: Fix failure to give back unlinked URBs

The syzkaller USB fuzzer identified a failure mode in which dummy-hcd
would never give back an unlinked URB.  This causes usb_kill_urb() to
hang, leading to WARNINGs and unkillable threads.

In dummy-hcd, all URBs are given back by the dummy_timer() routine as
it scans through the list of pending URBS.  Failure to give back URBs
can be caused by failure to start or early exit from the scanning
loop.  The code currently has two such pathways: One is triggered when
an unsupported bus transfer speed is encountered, and the other by
exhausting the simulated bandwidth for USB transfers during a frame.

This patch removes those two paths, thereby allowing all unlinked URBs
to be given back in a timely manner.  It adds a check for the bus
speed when the gadget first starts running, so that dummy_timer() will
never thereafter encounter an unsupported speed.  And it prevents the
loop from exiting as soon as the total bandwidth has been used up (the
scanning loop continues, giving back unlinked URBs as they are found,
but not transferring any more data).

Thanks to Andrey Konovalov for manually running the syzkaller fuzzer
to help track down the source of the bug.

Signed-off-by: Alan Stern <stern@rowland.harvard.edu>
Reported-and-tested-by:  <syzbot+d919b0f29d7b5a4994b9@syzkaller.appspotmail.com>
CC: <stable@vger.kernel.org>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>