block: Make protection interval calculation generic

Now that the protection interval has been detached from the sector size
we need to be able to handle sizes that are different from 4K and
512. Make the interval calculation generic.

Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Sagi Grimberg <sagig@mellanox.com>
Signed-off-by: Jens Axboe <axboe@fb.com>