[$] Guard pages for file-backed memory

One of the many new features packed into the 6.13 kernel release was guard
pages, a hardening mechanism that makes it possible to inject zero-access
pages into a process's address space in an efficient way. That feature
only supports anonymous (user-space data) pages, though. To make guard
pages more widely useful, Lorenzo Stoakes has put together a patch
set enabling the feature for file-backed pages as well; in the process,
he examined and resolved a long list of potential problems that extending
the feature could encounter. One potential problem was not on his list,
though.