selftests/mm: add tests for RO pinning vs fork()

Add a test suite (with 10 more sub-tests) to cover RO pinning against
fork() over uffd-wp.  It covers both:

  (1) Early CoW test in fork() when page pinned,
  (2) page unshare due to RO longterm pin.

They are:

  Testing wp-fork-pin on anon... done
  Testing wp-fork-pin on shmem... done
  Testing wp-fork-pin on shmem-private... done
  Testing wp-fork-pin on hugetlb... done
  Testing wp-fork-pin on hugetlb-private... done
  Testing wp-fork-pin-with-event on anon... done
  Testing wp-fork-pin-with-event on shmem... done
  Testing wp-fork-pin-with-event on shmem-private... done
  Testing wp-fork-pin-with-event on hugetlb... done
  Testing wp-fork-pin-with-event on hugetlb-private... done

CONFIG_GUP_TEST needed or they'll be skipped.

  Testing wp-fork-pin on anon... skipped [reason: Possibly CONFIG_GUP_TEST missing or unprivileged]

Note that the major test goal is on private memory, but no hurt to also run
all of them over shared because shared memory should work the same.

Link: https://lkml.kernel.org/r/[email protected]
Signed-off-by: Peter Xu <[email protected]>
Cc: Andrea Arcangeli <[email protected]>
Cc: Axel Rasmussen <[email protected]>
Cc: David Hildenbrand <[email protected]>
Cc: Mika Penttilä <[email protected]>
Cc: Mike Kravetz <[email protected]>
Cc: Nadav Amit <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

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