[PATCH] Fix bttv BUG() at video-buf.c:378

As found by Herbert Xu: the last v4l update broke bttv.  videobuf_iolock
was passed a vb that has just been filled with zeros. 

Fixed like this.

drivers/media/video/bttv-driver.c

@@ -2818,6 +2818,7 @@ static unsigned int bttv_poll(struct file *file, poll_table *wait)
 				up(&fh->cap.lock);
 				return POLLERR;
 			}
+			fh->cap.read_buf->memory = V4L2_MEMORY_USERPTR;
 			field = videobuf_next_field(&fh->cap);
 			if (0 != fh->cap.ops->buf_prepare(file,fh->cap.read_buf,field)) {
 				up(&fh->cap.lock);