[$] Composefs for integrity protection and data sharing

A read-only filesystem that will transparently share file data between disparatedirectory trees, while also providing integrity verification for the dataand the directory metadata, was recently posted as anRFCto the linux-kernel mailing list. Composefs was developedby Alexander Larsson (who posted it) and Giuseppe Scrivano for use by podman containers and OSTree (or "libostree" as itis now known) root directories, but there are likely others who want theabilities it provides. So far, there has been little response, either with feedback orcomplaints, but it is a small patch set (around 2K lines of code) andgenerally self-contained since it is a filesystem, so it would not be asurprise to see it appear in some upcoming kernel.