mm/swap: use swap_ops to register swap device's methods

From: Barry Song <baohua@kernel.org>

-v2:
 * lots of cleanup for patch 2/3: renaming, moving data
   structures, and using const properly
 * collected tags from Kairui, Nhat and Barry

-v1:
 https://lore.kernel.org/linux-mm/20260302104016.163542-1-...

This can simplify the code logic and benefit any new type of swap device
added later.

And also do renaming in this patchset:
-------
   file renaming:
   ---
   mm/page_io.c to mm/swap_io.c

   function renaming:
   ---
   swap_writepage_* to swap_write_folio_* in file mm/swap_io.c

Baoquan He (3):
  mm/swap: rename mm/page_io.c to mm/swap_io.c
  mm/swap: use swap_ops to register swap device's methods
  mm/swap_io.c: rename swap_writepage_* to swap_write_folio_*

 MAINTAINERS                 |   2 +-
 include/linux/swap.h        |   2 +
 mm/Makefile                 |   2 +-
 mm/swap.h                   |  12 ++++-
 mm/{page_io.c => swap_io.c} | 101 ++++++++++++++++++++----------------
 mm/swapfile.c               |   1 +
 mm/zswap.c                  |   3 +-
 7 files changed, 73 insertions(+), 50 deletions(-)
 rename mm/{page_io.c => swap_io.c} (89%)

-- 
2.39.3 (Apple Git-146)