md/md-bitmap: fix dm-raid max_write_behind setting

[ Upstream commit 2afe17794cfed5f80295b1b9facd66e6f65e5002 ]

It's supposed to be COUNTER_MAX / 2, not COUNTER_MAX.

Link: https://lore.kernel.org/linux-raid/20250524061320.370630-14-yukuai1@huaweicloud.com


Signed-off-by: Yu Kuai <yukuai3@huawei.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Hannes Reinecke <hare@suse.de>
Signed-off-by: Sasha Levin <sashal@kernel.org>