ALSA: usb-audio: fix rate_list memory leak

The array of sample rates is reallocated every time when opening
the PCM device, but was freed only once when unplugging the device.

Reported-by: "Alexander E. Patrakov" <patrakov@gmail.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Clemens Ladisch <clemens@ladisch.de>
Signed-off-by: Takashi Iwai <tiwai@suse.de>

sound/usb/pcm.c

@@ -788,6 +788,9 @@ static int snd_usb_pcm_check_knot(struct snd_pcm_runtime *runtime,
 	int count = 0, needs_knot = 0;
 	int err;
 
+	kfree(subs->rate_list.list);
+	subs->rate_list.list = NULL;
+
 	list_for_each_entry(fp, &subs->fmt_list, list) {
 		if (fp->rates & SNDRV_PCM_RATE_CONTINUOUS)
 			return 0;