mm/page_io: use sio->len for PSWPIN accounting in sio_read_complete()

sio_read_complete() uses sio->pages to account global PSWPIN vm events,
but sio->pages tracks the number of bvec entries (folios), not base pages.

While large folios cannot currently reach this path (SWP_FS_OPS and
SWP_SYNCHRONOUS_IO are mutually exclusive, and mTHP swap-in allocation is
gated on SWP_SYNCHRONOUS_IO), the accounting is semantically inconsistent
with the per-memcg path which correctly uses folio_nr_pages().

Use sio->len >> PAGE_SHIFT instead, which gives the correct base page
count since sio->len is accumulated via folio_size(folio).

Link: https://lore.kernel.org/20260402061408.36119-1-devnexen@gmail.com
Signed-off-by: David Carlier <devnexen@gmail.com>
Acked-by: David Hildenbrand (Arm) <david@kernel.org>
Cc: Baoquan He <bhe@redhat.com>
Cc: Chris Li <chrisl@kernel.org>
Cc: Kairui Song <kasong@tencent.com>
Cc: Kemeng Shi <shikemeng@huaweicloud.com>
Cc: NeilBrown <neil@brown.name>
Cc: Nhat Pham <nphamcs@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>