nvme-scsi: Consider LBA format in IO splitting calculation

The current command submission code uses a sector-based value when
considering the maximum number of blocks per command. With a
4k-formatted namespace and a command exceeding max hardware limits, this
calculation doesn't split IOs which should be split and fails in the
nvme layer. This patch fixes that calculation and enables IO splitting
in these circumstances.

Signed-off-by: Jon Derrick <jonathan.derrick@intel.com>
Reviewed-by: Jens Axboe <axboe@fb.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>