blktrace: add support for driver data

This patch adds the new api call blk_add_driver_data() to blktrace.
It allows to trace device driver-specific binary data.

Signed-off-by: Stefan Raspl <raspl@linux.vnet.ibm.com>
Signed-off-by: Martin Peschke <mp3@de.ibm.com>
Signed-off-by: Jens Axboe <jens.axboe@oracle.com>