ALSA: usb-audio: Cancel pending work at closing a MIDI substream

At closing a USB MIDI output substream, there might be still a pending
work, which would eventually access the rawmidi runtime object that is
being released.  For fixing the race, make sure to cancel the pending
work at closing.

Reported-by: syzbot+6912c9592caca7ca0e7d@syzkaller.appspotmail.com
Cc: <stable@vger.kernel.org>
Link: https://lore.kernel.org/r/000000000000e7e75005dfd07cf6@google.com
Link: https://lore.kernel.org/r/20220525131203.11299-1-tiwai@suse.deSigned-off-by: Takashi Iwai <tiwai@suse.de>

sound/usb/midi.c

@@ -1145,6 +1145,9 @@ static int snd_usbmidi_output_open(struct snd_rawmidi_substream *substream)
 
 static int snd_usbmidi_output_close(struct snd_rawmidi_substream *substream)
 {
+	struct usbmidi_out_port *port = substream->runtime->private_data;
+
+	cancel_work_sync(&port->ep->work);
 	return substream_open(substream, 0, 0);
 }