selftests/mm: run_vmtests.sh: fix hugetlb mem size calculation

The script calculates a mininum required size of hugetlb memories, but
it'll stop working with <1MB huge page sizes, reporting all zeros even if
huge pages are available.

In reality, the calculation doesn't really need to be as complicated
either.  Make it simpler and work for KB-level hugepages too.

[peterx@redhat.com: run_vmtests.sh: fix hugetlb mem size calculation]
  Link: https://lkml.kernel.org/r/20240403200324.1603493-1-peterx@redhat.com
Link: https://lkml.kernel.org/r/20240321215047.678172-1-peterx@redhat.com


Signed-off-by: Peter Xu <peterx@redhat.com>
Reviewed-by: David Hildenbrand <david@redhat.com>
Reviewed-by: Muchun Song <muchun.song@linux.dev>
Reviewed-by: Muhammad Usama Anjum <usama.anjum@collabora.com>
Tested-by: Ryan Roberts <ryan.roberts@arm.com>
Cc: Nico Pache <npache@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>