mm/zswap: remove SWP_SYNCHRONOUS_IO swapcache bypass workaround

mm/zswap: remove SWP_SYNCHRONOUS_IO swapcache bypass workaround

Since commit f1879e8a0c60 ("mm, swap: never bypass the swap cache even for
SWP_SYNCHRONOUS_IO"), all swap-in operations go through the swap cache,
including those from SWP_SYNCHRONOUS_IO devices like zram.  Which means
the workaround for swap cache bypassing introduced by commit 25cd241408a2
("mm: zswap: fix data loss on SWP_SYNCHRONOUS_IO devices") is no longer
needed.  Remove it, but keep the comments that are still helpful.

Link: https://lkml.kernel.org/r/20260202-zswap-syncio-cleanup-v1-1-86bb24a64521@tencent.com
Signed-off-by: Kairui Song <kasong@tencent.com>
Suggested-by: Yosry Ahmed <yosry.ahmed@linux.dev>
Reviewed-by: Barry Song <baohua@kernel.org>
Acked-by: Chris Li <chrisl@kernel.org>
Acked-by: Yosry Ahmed <yosry.ahmed@linux.dev>
Acked-by: Nhat Pham <nphamcs@gmail.com>
Reviewed-by: Chengming Zhou <chengming.zhou@linux.dev>
Cc: Baoquan He <bhe@redhat.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Kairui Song <kasong@tencent.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>