selftests/udmabuf: add a test to pin first before writing to memfd

selftests/udmabuf: add a test to pin first before writing to memfd

Unlike the existing tests, this new test will create a memfd (backed by
hugetlb) and pin the folios in it (a small subset) before writing/
populating it with data.  This is a valid use-case that invokes the
memfd_alloc_folio() kernel API and is expected to work unless there aren't
enough hugetlb folios to satisfy the allocation needs.

Link: https://lkml.kernel.org/r/20250618053415.1036185-4-vivek.kasireddy@intel.com
Signed-off-by: Vivek Kasireddy <vivek.kasireddy@intel.com>
Cc: Gerd Hoffmann <kraxel@redhat.com>
Cc: Steve Sistare <steven.sistare@oracle.com>
Cc: Muchun Song <muchun.song@linux.dev>
Cc: David Hildenbrand <david@redhat.com>
Cc: Oscar Salvador <osalvador@suse.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>