nvmet: add lba to sect conversion helpers

In this preparation patch, we add helpers to convert lbas to sectors &
sectors to lba. This is needed to eliminate code duplication in the ZBD
backend.

Use these helpers in the block device backend.

Signed-off-by: Chaitanya Kulkarni <chaitanya.kulkarni@wdc.com>
Reviewed-by: Damien Le Moal <damien.lemoal@wdc.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>