block: remove GENHD_FL_UP

Just check inode_unhashed on the whole device bdev inode instead,
and provide a helper to check for that information.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Link: https://lore.kernel.org/r/20210809064028.1198327-9-hch@lst.de


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