selftests/vm: anon_cow: add R/O longterm tests via gup_test

Let's trigger a R/O longterm pin on three cases of R/O mapped anonymous
pages:
* exclusive (never shared)
* shared (child still alive)
* previously shared (child no longer alive)

... and make sure that the pin is reliable: whatever we write via the page
tables has to be observable via the pin.

Link: https://lkml.kernel.org/r/[email protected]
Signed-off-by: David Hildenbrand <[email protected]>
Cc: Andrea Arcangeli <[email protected]>
Cc: Christoph von Recklinghausen <[email protected]>
Cc: Don Dutile <[email protected]>
Cc: Jason Gunthorpe <[email protected]>
Cc: John Hubbard <[email protected]>
Cc: Mike Rapoport <[email protected]>
Cc: Nadav Amit <[email protected]>
Cc: Peter Xu <[email protected]>
Cc: Shuah Khan <[email protected]>
Cc: Vlastimil Babka <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

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