selftests/vm: use memfd for hugepage-mremap test

For dropping the hugetlb mountpoint in run_vmtests.sh.  Cleaned it up a
little bit around the changed codes.

Link: https://lkml.kernel.org/r/20221014144013.94027-1-peterx@redhat.com


Signed-off-by: Peter Xu <peterx@redhat.com>
Cc: Axel Rasmussen <axelrasmussen@google.com>
Cc: Mike Kravetz <mike.kravetz@oracle.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>