selftests/vm: use memfd for hugepage-mmap test

This test was overlooked with a hard-coded mntpoint path in test when
we're removing the hugetlb mntpoint in commit 0796c7b8.  Fix it up so
the test can keep running.

Link: https://lkml.kernel.org/r/Y3aojfUC2nSwbCzB@x1n


Fixes: 0796c7b8 ("selftests/vm: drop mnt point for hugetlb in run_vmtests.sh")
Signed-off-by: Peter Xu <peterx@redhat.com>
Reported-by: Joel Savitz <jsavitz@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>