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

block/blk-settings.c

@@ -48,7 +48,7 @@ void blk_set_default_limits(struct queue_limits *lim)
 	lim->max_discard_sectors = 0;
 	lim->max_hw_discard_sectors = 0;
 	lim->max_secure_erase_sectors = 0;
-	lim->discard_granularity = 0;
+	lim->discard_granularity = 512;
 	lim->discard_alignment = 0;
 	lim->discard_misaligned = 0;
 	lim->logical_block_size = lim->physical_block_size = lim->io_min = 512;
@@ -309,6 +309,9 @@ void blk_queue_logical_block_size(struct request_queue *q, unsigned int size)
 
 	limits->logical_block_size = size;
 
+	if (limits->discard_granularity < limits->logical_block_size)
+		limits->discard_granularity = limits->logical_block_size;
+
 	if (limits->physical_block_size < size)
 		limits->physical_block_size = size;