selftests/vm: anon_cow: add liburing test cases

io_uring provides a simple mechanism to test long-term, R/W GUP pins
-- via fixed buffers -- and can be used to verify that GUP pins stay
in sync with the pages in the page table even if a page would
temporarily get mapped R/O or concurrent fork() could accidentially
end up sharing pinned pages with the child.

Note that this essentially re-introduces local_config support that was
removed recently in commit 6f83d6c7 ("Kselftests: remove support of
libhugetlbfs from kselftests").

[david@redhat.com: s/size_t/ssize_t/ on `cur', `total'.]
  Link: https://lkml.kernel.org/r/445fe1ae-9e22-0d1d-4d09-272231d2f84a@redhat.com
Link: https://lkml.kernel.org/r/20220927110120.106906-6-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>