Some filesystems may want to report different values depending on the path
within the filesystem, i.e. one mount is actually several filesystems. This
can be the case for a network filesystem exported by an unprivileged server
(e.g. sshfs).
This is now possible, thanks to David Howells "VFS: Permit filesystem to
perform statfs with a known root dentry" patch.
This change is backward compatible, so no need to change interface version.