selftests/vm: cow: basic COW tests for non-anonymous pages

Let's add basic tests for COW with non-anonymous pages in private
mappings: write access should properly trigger COW and result in the
private changes not being visible through other page mappings.

Especially, add tests for:
* Zeropage
* Huge zeropage
* Ordinary pagecache pages via memfd and tmpfile()
* Hugetlb pages via memfd

Fortunately, all tests pass.

Link: https://lkml.kernel.org/r/20221116102659.70287-3-david@redhat.com


Signed-off-by: David Hildenbrand <david@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>