Add a separate nvme_submit_user_cmd for commands that directly DMA
to or from userspace.  We'll add metadata support to that soon and
the common version would become too messy.
Signed-off-by: NChristoph Hellwig <hch@lst.de>
Signed-off-by: NKeith Busch <keith.busch@intel.com>
Signed-off-by: NJens Axboe <axboe@fb.com>