We should never see an inode marked as unlinked that's a subvolume root
(or a directory) in fsck, but even if we do it's not correct for fsck to
delete the subvolume: subvolumes are owned by dirents, and if we find a
dangling subvolume (not marked as unlinked) we want fsck to reattach it.
Signed-off-by: Kent Overstreet <kent.overstreet@gmail.com>