nvme: submit internal commands through the block layer

Use block layer queues with an internal cmd_type to submit internally
generated NVMe commands.  This both simplifies the code a lot and allow
for a better structure.  For example now the LighNVM code can construct
commands without knowing the details of the underlying I/O descriptors.
Or a future NVMe over network target could inject commands, as well as
could the SCSI translation and ioctl code be reused for such a beast.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jens Axboe <axboe@fb.com>