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

drivers/md/bcache/super.c

@@ -954,7 +954,7 @@ static int bcache_device_init(struct bcache_device *d, unsigned int block_size,
 	q->limits.max_segment_size	= UINT_MAX;
 	q->limits.max_segments		= BIO_MAX_VECS;
 	blk_queue_max_discard_sectors(q, UINT_MAX);
-	q->limits.discard_granularity	= 512;
+	q->limits.discard_granularity	= block_size;
 	q->limits.io_min		= block_size;
 	q->limits.logical_block_size	= block_size;
 	q->limits.physical_block_size	= block_size;