Use blk_rq_append_bio() internally instead of blk_rq_bio_prep()
so blk_rq_map_kern can be called multiple times, to map multiple
buffers.

This is in the effort to un-export blk_rq_append_bio()
Signed-off-by: NJames Bottomley <James.Bottomley@HansenPartnership.com>
Signed-off-by: NBoaz Harrosh <bharrosh@panasas.com>
Signed-off-by: NJens Axboe <jens.axboe@oracle.com>