Now that we open block devices as files we need to deal with the
realities that closing is a deferred operation. An operation on the
block device such as e.g., freeze, thaw, or removal that runs
concurrently with umount, tries to acquire a stable reference on the
holder. The holder might already be gone though. Make that reliable by
grabbing a passive reference to the holder during bdev_open() and
releasing it during bdev_release().

Fixes: f3a60882 ("bdev: open block device as files") # mainline only
Reported-by: Christoph Hellwig <hch@infradead.org>
Link: https://lore.kernel.org/r/ZfEQQ9jZZVes0WCZ@infradead.orgReviewed-by: Jan Kara <jack@suse.cz>
Reviewed-by: Christoph Hellwig <hch@infradead.org>
Tested-by: Yi Zhang <yi.zhang@redhat.com>
Reported-by: https://lore.kernel.org/r/CAHj4cs8tbDwKRwfS1=DmooP73ysM__xAb2PQc6XsAmWR+VuYmg@mail.gmail.com
Link: https://lore.kernel.org/r/20240315-freibad-annehmbar-ca68c375af91@braunerSigned-off-by: Christian Brauner <brauner@kernel.org>