selftests/mm: fix split_huge_page_test for folio_split() tests

selftests/mm: fix split_huge_page_test for folio_split() tests

PID_FMT does not have an offset field, so folio_split() tests are not
performed.  Add PID_FMT_OFFSET with an offset field and use it to perform
folio_split() tests.

Link: https://lkml.kernel.org/r/20250709012800.3225727-1-ziy@nvidia.com
Fixes: 80a5c494c89f ("selftests/mm: add tests for folio_split(), buddy allocator like split")
Signed-off-by: Zi Yan <ziy@nvidia.com>
Tested-by: Baolin Wang <baolin.wang@linux.alibaba.com>
Reviewed-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
Reviewed-by: Donet Tom <donettom@linux.ibm.com>
Tested-by : Donet Tom <donettom@linux.ibm.com>
Cc: Barry Song <baohua@kernel.org>
Cc: David Hildenbrand <david@redhat.com>
Cc: Dev Jain <dev.jain@arm.com>
Cc: Liam Howlett <liam.howlett@oracle.com>
Cc: Mariano Pache <npache@redhat.com>
Cc: Ryan Roberts <ryan.roberts@arm.com>
Cc: Shuah Khan <shuah@kernel.org>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>