V4L/DVB (7556): em28xx: fix locking on vidioc_s_fmt_cap

Currently, vidioc_s_fmt_cap is allowed even if streaming is running on some
other fh. This is likely to cause issues.

Block use of vidioc_s_fmt_cap if someone else has claimed access to the device.
Signed-off-by: Aidan Thornton <makosoft@googlemail.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>

drivers/media/video/em28xx/em28xx-video.c

@@ -947,6 +947,12 @@ static int vidioc_s_fmt_cap(struct file *file, void *priv,
 		goto out;
 	}
 
+	if (dev->stream_on && !fh->stream_on) {
+		em28xx_errdev("%s device in use by another fh\n", __func__);
+		rc = -EBUSY;
+		goto out;
+	}
+
 	/* set new image size */
 	dev->width = f->fmt.pix.width;
 	dev->height = f->fmt.pix.height;