s390: supplement for ptdesc conversion

After commit 6326c26c ("s390: convert various pgalloc functions to use
ptdescs"), there are still some positions that use page->{lru, index}
instead of ptdesc->{pt_list, pt_index}.  In order to make the use of
ptdesc->{pt_list, pt_index} clearer, it would be better to convert them as
well.

[zhengqi.arch@bytedance.com: fix build failure]
  Link: https://lkml.kernel.org/r/20240305072154.26168-1-zhengqi.arch@bytedance.com
Link: https://lkml.kernel.org/r/04beaf3255056ffe131a5ea595736066c1e84756.1709541697.git.zhengqi.arch@bytedance.com


Signed-off-by: Qi Zheng <zhengqi.arch@bytedance.com>
Cc: Christian Borntraeger <borntraeger@linux.ibm.com>
Cc: Janosch Frank <frankja@linux.ibm.com>
Cc: Claudio Imbrenda <imbrenda@linux.ibm.com>
Cc: David Hildenbrand <david@redhat.com>
Cc: Hugh Dickins <hughd@google.com>
Cc: Matthew Wilcox (Oracle) <willy@infradead.org>
Cc: Mike Rapoport (IBM) <rppt@kernel.org>
Cc: Muchun Song <muchun.song@linux.dev>
Cc: Vishal Moola (Oracle) <vishal.moola@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>