NVMe: fix user commands

The commands that may be executed by all users within a system are directed to
use the mq queues, while the priviledged commands are routed through the
self-contained admin queue.
NVMe: merge to previous
NVMe: remove references to bio.h

To allow us to completely go away from a bio-based driver, we remove the last
references used in submit_bio_queue and replace them with their request-based
equivalents.
merge with the one before
NVMe: let nvme_submit_rq_queue return status

Rearrange the logic to remove switch statement in nvme_queue_request and
simplify return path.