Bart did this patch, I changed it a bit. Basically it cleans the mapping
interface up a bit, and adds a little helper to set up the request from
the bio.

In addition, it fixes a long standing bug where bio_map_user() would
call blk_queue_bounce() without the direction bit being set, auch.

 - Abstract out bio request preparation
 - Have bio_map_user() set data direction (fixes bug where blk_queue_bounce()
   is called without it set)
 - Split bio_map_user() in two