tools: broken assumption in mdflush

Since kernel commit 309dca309fc ("block: store a block_device pointer in struct bio") struct bio points again to a block_device and not to a gendisk directly. However mdflush is looking at the presence or not of the bio_dev macro to check whether to get the gendisk directly from the bio or not, which doesn't work anymore since the bio_dev macro still exists.

Start by locating mdflush and inspecting its bio_dev conditional, then compare that assumption with the current Linux struct bio and block_device relationship described in the issue. Done means mdflush obtains the gendisk through the current bio representation rather than using the continued existence of bio_dev as its feature check.

Written by the indexing model from the issue text.