V4L: buf_qbuf: fix videobuf_queue->stream corruption and lockup

We are doing ->buf_prepare(buf) before adding buf to q->stream list. This
means that videobuf_qbuf() should not try to re-add a STATE_PREPARED buffer.

(cherry picked from commit 419dd837)
Signed-off-by: Oleg Nesterov <oleg@tv-sign.ru>
Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>
Signed-off-by: Michael Krufky <mkrufky@linuxtv.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

drivers/media/video/video-buf.c

@@ -700,6 +700,7 @@ videobuf_qbuf(struct videobuf_queue *q,
 		goto done;
 	}
 	if (buf->state == STATE_QUEUED ||
+	    buf->state == STATE_PREPARED ||
 	    buf->state == STATE_ACTIVE) {
 		dprintk(1,"qbuf: buffer is already queued or active.\n");
 		goto done;