Linux 5.12 compat: bio->bi_disk member moved

The struct bio member bi_disk was moved underneath a new member named
bi_bdev. So all attempts to reference bio->bi_disk need to now become
bio->bi_bdev->bd_disk.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Coleman Kane <ckane@colemankane.org>
Closes #11639