mm: swap: make some count_mthp_stat() call-sites be THP-agnostic.

In commit 246d3aa3 ("mm: cleanup count_mthp_stat() definition"), Ryan
Roberts has pointed out the merits of mm code that does not require THP,
to be compile-able without requiring THP ifdefs.  As a step in that
direction, he has moved count_mthp_stat() to be always defined, resolving
to a no-op if THP is not defined.

Barry Song referred me to Ryan's commit when I was working on the "mm:
zswap swap-out of large folios" patch-series [1].

This patch propagates the benefits of the above change to page_io.c and
vmscan.c.  As a result, there is one less reason to have the ifdef THP in
these code sections.

[1]: https://patchwork.kernel.org/project/linux-mm/list/?series=894347

Link: https://lkml.kernel.org/r/20241002225822.9006-1-kanchana.p.sridhar@intel.com


Signed-off-by: Kanchana P Sridhar <kanchana.p.sridhar@intel.com>
Reviewed-by: Nhat Pham <nphamcs@gmail.com>
Cc: Chengming Zhou <chengming.zhou@linux.dev>
Cc: "Huang, Ying" <ying.huang@intel.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Ryan Roberts <ryan.roberts@arm.com>
Cc: Wajdi Feghali <wajdi.k.feghali@intel.com>
Cc: Yosry Ahmed <yosryahmed@google.com>
Cc: Barry Song <21cnbao@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>