ALSA: Add a reference counter to card instance

For more strict protection for wild disconnections, a refcount is
introduced to the card instance, and let it up/down when an object is
referred via snd_lookup_*() in the open ops.

The free-after-last-close check is also changed to check this refcount
instead of the empty list, too.

Reported-by: Matthieu CASTET <matthieu.castet@parrot.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Takashi Iwai <tiwai@suse.de>

sound/core/oss/pcm_oss.c

@@ -2457,6 +2457,8 @@ static int snd_pcm_oss_open(struct inode *inode, struct file *file)
       __error2:
       	snd_card_file_remove(pcm->card, file);
       __error1:
+	if (pcm)
+		snd_card_unref(pcm->card);
 	return err;
 }