[media] media: dmxdev: remove dvb_ringbuffer_flush() on writer side

In dvb_ringbuffer lock-less synchronizationof reader and writer threads is done
with separateread and write pointers. Sincedvb_ringbuffer_flush() modifies the
read pointer, this function must not be called from the writer thread.
This patch removes the dvb_ringbuffer_flush() calls in the dmxdev ringbuffer
write functions, this fixes Oopses "Unable to handle kernel paging request"
I could observe for the call chaindvb_demux_read ->dvb_dmxdev_buffer_read ->
dvb_ringbuffer_read_user -> __copy_to_user (the reader side of the ringbuffer).
The flush calls at the write side are not necessary anyway since ringbuffer_flush
is also called in dvb_dmxdev_buffer_read() when an error condition is set in the
ringbuffer.
This patch should also be applied to stable kernels.

Signed-off-by: Soeren Moch <[e-mail]>
CC: <[e-mail]>
Reviewed-by: Sakari Ailus <[e-mail]>
Signed-off-by: Mauro Carvalho Chehab <[e-mail]>

drivers/media/dvb-core/dmxdev.c

···
 		ret = dvb_dmxdev_buffer_write(&dmxdevfilter->buffer, buffer2,
 					      buffer2_len);
 	}
-	if (ret < 0) {
-		dvb_ringbuffer_flush(&dmxdevfilter->buffer);
+	if (ret < 0)
 		dmxdevfilter->buffer.error = ret;
-	}
 	if (dmxdevfilter->params.sec.flags & DMX_ONESHOT)
 		dmxdevfilter->state = DMXDEV_STATE_DONE;
 	spin_unlock(&dmxdevfilter->dev->lock);
···
 	ret = dvb_dmxdev_buffer_write(buffer, buffer1, buffer1_len);
 	if (ret == buffer1_len)
 		ret = dvb_dmxdev_buffer_write(buffer, buffer2, buffer2_len);
-	if (ret < 0) {
-		dvb_ringbuffer_flush(buffer);
+	if (ret < 0)
 		buffer->error = ret;
-	}
 	spin_unlock(&dmxdevfilter->dev->lock);
 	wake_up(&buffer->queue);
 	return 0;