nvme: split __nvme_submit_sync_cmd

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: Christoph Hellwig <hch@lst.de>
Signed-off-by: Keith Busch <keith.busch@intel.com>
Signed-off-by: Jens Axboe <axboe@fb.com>