libosd: Use new blk_rq_map_kern

Now that blk_rq_map_kern will append the buffer onto the
request we can use it easily for adding extra segments
(eg. attributes)

This patch is dependent on a block layer patch titled:
   [BLOCK] allow blk_rq_map_kern to append to requests

Signed-off-by: Boaz Harrosh <bharrosh@panasas.com>
Signed-off-by: Jens Axboe <jens.axboe@oracle.com>