block: default the discard granularity to sector size

Current the discard granularity defaults to 0 and must be initialized by
any driver that wants to support discard.  Default to the sector size
instead, which is the smallest possible value, and a very useful default.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Link: https://lore.kernel.org/r/20231228075545.362768-4-hch@lst.de


Signed-off-by: Jens Axboe <axboe@kernel.dk>