Revert "FROMGIT: userfaultfd/selftests: use memfd_create for shmem test type"

This reverts commit f8bed3c813cb21c9c576d42068994e0199773f96
as an updated version of the patch-set will be merged later.

Signed-off-by: Lokesh Gidra <lokeshgidra@google.com>
Bug: 187930641
Change-Id: I808f6c58429c83f03eae2ad2cbe65a232b2505c8