userfaultfd/selftests: clean up hugetlb allocation code

The message for commit f5c73297 ("userfaultfd/selftests: fix hugetlb
area allocations") says there is no need to create a hugetlb file in the
non-shared testing case.  However, the commit did not actually change
the code to prevent creation of the file.

While it is technically true that there is no need to create and use a
hugetlb file in the case of non-shared-testing, it is useful.  This is
because 'hole punching' of a hugetlb file has the potentially incorrect
side effect of also removing pages from private mappings.  The
userfaultfd test relies on this side effect for removing pages from the
destination buffer during rounds of stress testing.

Remove the incomplete code that was added to deal with no hugetlb file.
Just keep the code that prevents reserves from being created for the
destination area.

Link: https://lkml.kernel.org/r/20220104021729.111006-1-mike.kravetz@oracle.com


Signed-off-by: Mike Kravetz <mike.kravetz@oracle.com>
Reviewed-...