ALSA: xen-front: Do not use stream buffer size before it is set

This fixes the regression introduced while moving to Xen shared
buffer implementation.

Fixes: 58f9d806 ("ALSA: xen-front: Use Xen common shared buffer implementation")
Reviewed-by: Juergen Gross <jgross@suse.com>
Signed-off-by: Oleksandr Andrushchenko <oleksandr_andrushchenko@epam.com>
Cc: <stable@vger.kernel.org> # v5.0+
Signed-off-by: Takashi Iwai <tiwai@suse.de>

sound/xen/xen_snd_front_alsa.c

@@ -441,7 +441,7 @@ static int shbuf_setup_backstore(struct xen_snd_front_pcm_stream_info *stream,
 {
 	int i;
 
-	stream->buffer = alloc_pages_exact(stream->buffer_sz, GFP_KERNEL);
+	stream->buffer = alloc_pages_exact(buffer_sz, GFP_KERNEL);
 	if (!stream->buffer)
 		return -ENOMEM;