limit deep scount (folder contents and size) to one filesystem

Closes https://github.com/mate-desktop/caja/issues/194
Thanks to monsta <monsta@inbox.ru> to have suggested this solution

Original GNOME commit:
https://git.gnome.org/browse/nautilus/commit/?id=a645da5f1043c59203fd194fe85b6976d75d2ece

When getting the size of the root directory, nautilus was descending
into other filesystems including /proc, causing it to report nonsensical
sizes.
Store the fsid of the starting directory, and do not recurse into
other mount points.
https://bugzilla.gnome.org/show_bug.cgi?id=629394
https://bugs.launchpad.net/ubuntu/+source/nautilus/+bug/585472