Use the namespace's block format for reporting the max transfer length.

Max unmap count is left as-is since NVMe doesn't provide a max, so the
value the driver provided the block layer is valid for any format.
Reported-by: Martin K. Petersen <martin.petersen@oracle.com>
Signed-off-by: Keith Busch <keith.busch@intel.com>
Signed-off-by: Jens Axboe <axboe@fb.com>