This patch allows you to map a request with user data for io, similarly
to what you can do with bio_map_user() already to a bio. However, this
goes one step further and populates the request so the user only has to
fill in the cdb (almost) and put it on the queue for execution. Patch
converts sg_io() to use it, next patch I'll send adapts cdrom layer to
use it for zero copy cdda dma extraction.