rbd: increase io_opt again

Commit 16d80c5 ("rbd: set io_min, io_opt and discard_granularity to
alloc_size") lowered the io_opt size for rbd from objset_bytes which is
4MB for typical setup to alloc_size which is typically 64KB.

The commit mostly talks about discard behavior and does mention io_min
in passing.  Reducing io_opt means reducing the readahead size, which
seems counter-intuitive given that rbd currently abuses the user
max_sectors setting to actually increase the I/O size.  Switch back
to the old setting to allow larger reads (the readahead size despite it's
name actually limits the size of any buffered read) and to prepare
for using io_opt in the max_sectors calculation and getting drivers out
of the business of overriding the max_user_sectors value.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Acked-by: Ilya Dryomov <idryomov@gmail.com>
Reviewed-by: Martin K. Petersen <martin.petersen@oracle.com>
Link: https://lore.kernel.org/r/20240531074837.1648501-4-hch@lst.de
Signed-off-by: Jens Axboe <axboe@kernel.dk>

drivers/block/rbd.c

@@ -4955,8 +4955,8 @@ static int rbd_init_disk(struct rbd_device *rbd_dev)
 	struct queue_limits lim = {
 		.max_hw_sectors		= objset_bytes >> SECTOR_SHIFT,
 		.max_user_sectors	= objset_bytes >> SECTOR_SHIFT,
+		.io_opt			= objset_bytes,
 		.io_min			= rbd_dev->opts->alloc_size,
-		.io_opt			= rbd_dev->opts->alloc_size,
 		.max_segments		= USHRT_MAX,
 		.max_segment_size	= UINT_MAX,
 	};