f2fs: drop usage of folio_index

f2fs: drop usage of folio_index

folio_index is only needed for mixed usage of page cache and swap cache,
for pure page cache usage, the caller can just use folio->index instead.

It can't be a swap cache folio here.  Swap mapping may only call into fs
through `swap_rw` but f2fs does not use that method for swap.

Link: https://lkml.kernel.org/r/20250430181052.55698-4-ryncsn@gmail.com
Signed-off-by: Kairui Song <kasong@tencent.com>
Reviewed-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Reviewed-by: David Hildenbrand <david@redhat.com>
Reviewed-by: Chao Yu <chao@kernel.org>
Cc: Jaegeuk Kim <jaegeuk@kernel.org>
Cc: Chris Li <chrisl@kernel.org>
Cc: Chris Mason <clm@fb.com>
Cc: Christian Brauner <brauner@kernel.org>
Cc: David Sterba <dsterba@suse.com>
Cc: "Huang, Ying" <ying.huang@linux.alibaba.com>
Cc: Hugh Dickins <hughd@google.com>
Cc: Joanne Koong <joannelkoong@gmail.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Josef Bacik <josef@toxicpanda.com>
Cc: Miklos Szeredi <miklos@szeredi.hu>
Cc: Nhat Pham <nphamcs@gmail.com>
Cc: Qu Wenruo <wqu@suse.com>
Cc: Yosry Ahmed <yosryahmed@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>