[media] cx18: Fix a sleep-in-atomic bug in snd_cx18_pcm_hw_free

The driver may sleep under a spin lock, and the function call path is:
snd_cx18_pcm_hw_free (acquire the lock by spin_lock_irqsave)
  vfree --> may sleep

To fix it, the "substream->runtime->dma_area" is passed to a temporary
value, and mark it NULL when holding the lock. The memory is freed by
vfree through the temporary value outside the lock holding.
Signed-off-by: Jia-Ju Bai <baijiaju1990@163.com>
[hans.verkuil@cisco.com: removed unnecessary 'if (dma_area)']
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>

drivers/media/pci/cx18/cx18-alsa-pcm.c

@@ -257,14 +257,16 @@ static int snd_cx18_pcm_hw_free(struct snd_pcm_substream *substream)
 {
 	struct snd_cx18_card *cxsc = snd_pcm_substream_chip(substream);
 	unsigned long flags;
+	unsigned char *dma_area = NULL;
 
 	spin_lock_irqsave(&cxsc->slock, flags);
 	if (substream->runtime->dma_area) {
 		dprintk("freeing pcm capture region\n");
-		vfree(substream->runtime->dma_area);
+		dma_area = substream->runtime->dma_area;
 		substream->runtime->dma_area = NULL;
 	}
 	spin_unlock_irqrestore(&cxsc->slock, flags);
+	vfree(dma_area);
 
 	return 0;
 }