mm: memfd_luo: always dirty all folios

In the Linux kernel, the following vulnerability has been resolved:

mm: memfd_luo: always dirty all folios

A dirty folio is one which has been written to. A clean folio is its
opposite. Since a clean folio has no user data, it can be freed under
memory pressure.

memfd preservation with LUO saves the flag at preserve(). This is
problematic. The folio might get dirtied later. Saving it at freeze()
also doesn't work, since the dirty bit from PTE is normally synced at
unmap and there might still be mappings of the file at freeze().

To see why this is a problem, say a folio is clean at preserve, but gets
dirtied later. The serialized state of the folio will mark it as clean.
After retrieve, the next kernel will see the folio as clean and might try
to reclaim it under memory pressure. This will result in losing user
data.

Mark all folios of the file as dirty, and always set the
MEMFD_LUO_FOLIO_DIRTY flag. This comes with the side effect of making all
clean folios un-reclaimable. This is a cost that has to be paid for
participants of live update. It is not expected to be a common use case
to preserve a lot of clean folios anyway.

Since the value of pfolio->flags is a constant now, drop the flags
variable and set it directly.