bcache: discard_granularity should not be smaller than a sector

Just like all block I/O, discards are in units of sectors.  Thus setting a
smaller than sector size discard limit in case of > 512 byte sectors in
bcache doesn't make sense.  Always set the discard granularity to 512
bytes instead.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Link: https://lore.kernel.org/r/20231228075545.362768-3-hch@lst.de


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