The majority of blockdev filesystems, which do not have a UUID in their
on-disk format, derive f_fsid of statfs(2) from bdev->bd_dev.

Use the same practice for freevxfs.

This will allow reporting fanotify events with fanotify_event_info_fid.
Signed-off-by: Amir Goldstein <amir73il@gmail.com>
Link: https://lore.kernel.org/r/20231024121457.3014063-1-amir73il@gmail.comReviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Christian Brauner <brauner@kernel.org>