nvme: Introduce nvme_lba_to_sect()

Introduce the new helper function nvme_lba_to_sect() to convert a device
logical block number to a 512B sector number. Use this new helper in
obvious places, cleaning up the code.

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>