V4L/DVB (6609): Re-adds lock safe videobuf_read_start

videobuf_dvb needs videobuf_read_start. The EXPORT_SYMBOL_GPL() were removed by
a previous patch. However, videobuf_dvb needs this.

This patch re-adds videobuf_read_start, doing the proper lock.
Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>

drivers/media/video/videobuf-core.c

@@ -806,7 +806,7 @@ ssize_t videobuf_read_one(struct videobuf_queue *q,
 }
 
 /* Locking: Caller holds q->lock */
-int videobuf_read_start(struct videobuf_queue *q)
+int __videobuf_read_start(struct videobuf_queue *q)
 {
 	enum v4l2_field field;
 	unsigned long flags=0;
@@ -862,6 +862,17 @@ static void __videobuf_read_stop(struct videobuf_queue *q)
 	
 }
 
+int videobuf_read_start(struct videobuf_queue *q)
+{
+	int rc;
+
+	mutex_lock(&q->lock);
+	rc = __videobuf_read_start(q);
+	mutex_unlock(&q->lock);
+
+	return rc;
+}
+
 void videobuf_read_stop(struct videobuf_queue *q)
 {
 	mutex_lock(&q->lock);
@@ -1058,6 +1069,7 @@ EXPORT_SYMBOL_GPL(videobuf_dqbuf);
 EXPORT_SYMBOL_GPL(videobuf_streamon);
 EXPORT_SYMBOL_GPL(videobuf_streamoff);
 
+EXPORT_SYMBOL_GPL(videobuf_read_start);
 EXPORT_SYMBOL_GPL(videobuf_read_stop);
 EXPORT_SYMBOL_GPL(videobuf_stop);
 EXPORT_SYMBOL_GPL(videobuf_read_stream);