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

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/20220927110120.106906-8-david@redhat.com
Signed-off-by: David Hildenbrand <david@redhat.com>
Cc: Andrea Arcangeli <aarcange@redhat.com>
Cc: Christoph von Recklinghausen <crecklin@redhat.com>
Cc: Don Dutile <ddutile@redhat.com>
Cc: Jason Gunthorpe <jgg@nvidia.com>
Cc: John Hubbard <jhubbard@nvidia.com>
Cc: Mike Rapoport <rppt@kernel.org>
Cc: Nadav Amit <namit@vmware.com>
Cc: Peter Xu <peterx@redhat.com>
Cc: Shuah Khan <shuah@kernel.org>
Cc: Vlastimil Babka <vbabka@suse.cz>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>