mm/swapfile: use plist_for_each_entry in __folio_throttle_swaprate

mm/swapfile: use plist_for_each_entry in __folio_throttle_swaprate

The loop breaks immediately after finding the first swap device and
never modifies the list. Replace plist_for_each_entry_safe() with
plist_for_each_entry() and remove the unused next variable.

Link: https://lkml.kernel.org/r/20251127100303.783198-3-youngjun.park@lge.com
Signed-off-by: Youngjun Park <youngjun.park@lge.com>
Reviewed-by: Baoquan He <bhe@redhat.com>
Acked-by: Kairui Song <kasong@tencent.com>
Acked-by: Chris Li <chrisl@kernel.org>
Cc: Barry Song <baohua@kernel.org>
Cc: Kemeng Shi <shikemeng@huaweicloud.com>
Cc: Nhat Pham <nphamcs@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>