V4L/DVB: cx18: Add missing serialization locks to cx18-dvb

Operations that manipulates the state of the encoder or global instance flags
and variables should be serialized.  This adds some serialization locks that
have been missing from cx18-dvb.  Since the DVB part of the CX23418 doesn't
affect the encoder, no one would likely have ever noticed a race in changing
card instance variables.
Signed-off-by: Andy Walls <awalls@radix.net>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>

drivers/media/video/cx18/cx18-dvb.c

@@ -260,6 +260,7 @@ static int cx18_dvb_start_feed(struct dvb_demux_feed *feed)
 	mutex_lock(&stream->dvb.feedlock);
 	if (stream->dvb.feeding++ == 0) {
 		CX18_DEBUG_INFO("Starting Transport DMA\n");
+		mutex_lock(&cx->serialize_lock);
 		set_bit(CX18_F_S_STREAMING, &stream->s_flags);
 		ret = cx18_start_v4l2_encode_stream(stream);
 		if (ret < 0) {
@@ -268,6 +269,7 @@ static int cx18_dvb_start_feed(struct dvb_demux_feed *feed)
 			if (stream->dvb.feeding == 0)
 				clear_bit(CX18_F_S_STREAMING, &stream->s_flags);
 		}
+		mutex_unlock(&cx->serialize_lock);
 	} else
 		ret = 0;
 	mutex_unlock(&stream->dvb.feedlock);
@@ -291,7 +293,9 @@ static int cx18_dvb_stop_feed(struct dvb_demux_feed *feed)
 		mutex_lock(&stream->dvb.feedlock);
 		if (--stream->dvb.feeding == 0) {
 			CX18_DEBUG_INFO("Stopping Transport DMA\n");
+			mutex_lock(&cx->serialize_lock);
 			ret = cx18_stop_v4l2_encode_stream(stream, 0);
+			mutex_unlock(&cx->serialize_lock);
 		} else
 			ret = 0;
 		mutex_unlock(&stream->dvb.feedlock);