block: floor the discard granularity to the physical block size

Discarding less than a physical block doesn't make sense.  This fixes
the existing behavior for zram before the recent changes to default
the discard granularity to the logical block size, and is also a
generally useful sanity check.

Fixes: 3753039d ("zram: use the default discard granularity")
Reported-by: Sergey Senozhatsky <senozhatsky@chromium.org>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Link: https://lore.kernel.org/r/20240103081622.508754-1-hch@lst.deSigned-off-by: Jens Axboe <axboe@kernel.dk>

block/blk-settings.c

@@ -342,6 +342,9 @@ void blk_queue_physical_block_size(struct request_queue *q, unsigned int size)
 	if (q->limits.physical_block_size < q->limits.logical_block_size)
 		q->limits.physical_block_size = q->limits.logical_block_size;
 
+	if (q->limits.discard_granularity < q->limits.physical_block_size)
+		q->limits.discard_granularity = q->limits.physical_block_size;
+
 	if (q->limits.io_min < q->limits.physical_block_size)
 		q->limits.io_min = q->limits.physical_block_size;
 }