nvme: Cleanup and rename nvme_block_nr()

Rename nvme_block_nr() to nvme_sect_to_lba() and use SECTOR_SHIFT
instead of its hard coded value 9. Also add a comment to decribe this
helper.

Reviewed-by: Johannes Thumshirn <jthumshirn@suse.de>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Damien Le Moal <damien.lemoal@wdc.com>
Signed-off-by: Keith Busch <kbusch@kernel.org>
Signed-off-by: Jens Axboe <axboe@kernel.dk>