nvme/scsi: don't rely on BLK_MAX_CDB

The NVMe SCSI emulation doesn't use BLOCK_PC requests, so BLK_MAX_CDB
doesn't have a meaning for it.  Instead opencode the value of 16
and refactor the code a bit so that related checks are next to each
other and we only need to use the value in one place.

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