selftests/mm: uffd-unit-tests support for hugepages > 2M

uffd-unit-tests uses a memory area with a fixed 32M size.  Then it
calculates the number of pages by dividing by page_size, which itself is
either the base page size or the PMD huge page size depending on the test
config.  For the latter, we end up with nr_pages=1 for arm64 16K base
pages, and nr_pages=0 for 64K base pages.  This doesn't end well.

So let's make the 32M size a floor and also ensure that we have at least 2
pages given the PMD size.  With this change, the tests pass on arm64 64K
base page size configuration.

Link: https://lkml.kernel.org/r/20250318174343.243631-2-ryan.roberts@arm.com


Signed-off-by: Ryan Roberts <ryan.roberts@arm.com>
Acked-by: Peter Xu <peterx@redhat.com>
Acked-by: Rafael Aquini <raquini@redhat.com>
Cc: Shuah Khan <shuah@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>