This call should be a sufficiently close approximation of what happens
when a filesystem is unmounted and remounted. To be more specific, it
should test that the data that was in the page cache is the same data
that ends up on a stable storage or in a filesystem's internal cache,
if any.
This will catch the cases where a page with modified data is marked as
a clean page for whatever reason.
While there, make logging of the special events (open+close before
plus invalidation now) more generic and slightly better than the previous
hack.
MFC after: 10 days
fb4c145f0e